Effector CD4 T cells in HIV and AIDS

Several effector CD4 T cells are frequently mentioned in HIV pathogenesis and AIDS.  Here, I will discuss their roles in general protective immunity against microbial pathogens including HIV.  Current paradigms on the generation and function of effector CD4 T cells will be discussed.  I will also discuss whether our current paradigms on this issue is adequate to explain the HIV pathogenesis and AIDS.

Highlights:

• Define effector CD4 T cells
• Loss of effector memory CD4 T cells in HIV infected individuals
• Current paradigms on the generation of effector CD4 T cells
• Current paradigms on the role of effector CD4 T cells in protective immunity

Effector CD4 T cells:
Effector CD4 T cells are pre-differentiated CD4 T cells that can perform a variety of effector function when challenged by a cell specific antigen  Several types of effector CD4 T cells, such as Th1, Th2, Th17, TFH and Treg cells, perform various effector functions.  These cells distinguish themselves by their characteristic cytokines produced during proliferation and their cell specific transcription factors for each type of effector T cells.  To become a specific effector CD4 T cells, they require a specific cytokine during activation/differentiation steps. Details of cytokines required to become each effector T cells and cytokines produced by each of them can be found in many other places, including free web sites, such as wikipedia. (for example, Th17 cells in wikipedia: http://en.wikipedia.org/wiki/T_helper_17_cell).

Loss of effector CD4 T cells in HIV infected individuals:
The specific loss of effector CD4 T cells during and after HIV infection.
The information on the loss of a specific type of effector T cells in HIV infected individuals or selective susceptibility of CD4 T cells by HIV infection will provide an invaluable information not only on the origin of effector T cells, but also on HIV pathogenesis.  The loss of Th17 cells in HIV (or SIV) infected individuals (non-human primates) is well established  by many groups.  The loss of IFN-g producing cells in acute SIV infection (Dandekar's group in Nature Medicine) and that of Treg cells found in LP of gut, but not those in lymphoid organs and blood (Chase et al.  by Siliciano group JVI 2007 Severe depletion of CD4+ CD25+ regulatory T cells from the intestinal lamina propria but not peripheral blood or lymph nodes during acute simian immunodeficiency virus infection) are not well known.
In addition, CCR5+CCR6+ IFN-g producing Th1 or CCR5+CCR6+ IL-4 producing Th2 cells are susceptible to HIV infection, while conventional Th1 or Th2 cells (both are CCR6-CCR5-) are resistant to HIV infection (Rafick Sekaly;s group in JI. 2010 Peripheral blood CCR4+CCR6+ and CXCR3+CCR6+CD4+ T cells are highly permissive toHIV-1 infection).  This suggests that even the IFN-g or IL-4 producing Th1 or Th2 cells are two different types with regards to HIV susceptibility and expressions of chemokine receptors.

Th17: It has been established that certain effector CD4 T cells are selectively affected by an HIV infection.  The loss of Th17 cells during HIV infection is the first one with wide acceptance.  A  special issue was published on the subject of the gut and Th17 in HIV infection (13 reviews and an editorial in an issue of Current Opinions in HIV and AIDS: The gut and Th17 in HIV infection:  http://journals.lww.com/co-hivandaids/toc/2010/03000 )
Even with these extensive research by many investigators, there is no clear explanation why Th17 cells are missing in HIV infected individuals.  Not a single article questioned about the origin of Th17 cells could be CCR5+CD4 T cells that are susceptible to R5-tropic HIV (HST), and still don't.  I will offer my view in a separate discussion, which is different from all of them later.

TFH: Is there a loss of TFH cells in HIV infected individuals, which has a very significant impact on HIV vaccine development utilizing BnAb?  This issue requires a special attention in HIV pathogenesis and HIV vaccine development.  However, it has not been studied extensively yet due to our understanding of TFH is still an infant stage.  The presence and the role of TFH in HIV infected individuals is a very important issue, in particular, since the effector function mediated by TFH is critical to generate high affinity antibody generation.  TFH is involved in several steps of B cell maturation, including an isotype switching followed by an affinity maturation.  Early part of the B cell maturation can occur at an extrafollicular stage even before generating germinal center, while affinity maturation occurs at the germinal center after formation.  Both steps require a transcription factor Bcl-6 expressing TFH cells, although they could be two different different types of TFH cells.  You can imagine that loss of an early emerging TFH cells could impact severely on the generation of broad neutralizing antibody (BnAb).  Are they affected by HIV infection?  There isn't any study directly asking that specific question, unlike Th17 cells in HIV infection.  However, a study by Klatt et al., (SIV infection of rhesus macaques results in dysfunctional T- and B-cell responses to neo and recall Leishmania major vaccination. Klatt NR, Vinton CL, Lynch RM, Canary LA, Ho J, Darrah PA, Estes JD, Seder RA, Moir SL,Brenchley JM. Blood. 2011 Nov 24;118(22):5803-12. Epub 2011 Sep 29.) suggests that there is a possibility that TFH cells could be missing in action during SIV infected non-human primates.  If that will be the case, an HIV vaccine strategy generating the broad neutralizing antibody against the HIV env protein (gp120) needs to be evaluated with caution.  It will also explain why it has been so difficult to generate neutralizing antibodies against HIV even after many well-managed trials.  My inclination on this issue is that early appearing TFH cells, extrafollicular TFH, could be derived (differentiated) from the HST.  If that will be the case, loss of HST by HIV infection, will severely hamper the generation of high affinity neutralizing antibodies, especially an isotype switched, IgG type, antibody.  It will be interesting to see whether the secretion of IgA type antibody also requires extrafollicular TFH cells or not, since accumulation of IgA type antibody negatively affected HIV vaccine efficacy in RV144 trial.

Th1: Th1 cells can be generated in vitro (or ex vivo meaning outside the body in an artificial tissue culture system) by stimulating naive CD4 T cells in the presence of IL-12.  After 3 to 5 days of culture in conditioned media, CD4 T cells that were differentiated into Th1 cells produce IFN-g when re-stimulated.  It is a very slow process inside the body, since there are very few antigen specific CD4 T cells.  Therefore, it usually takes several weeks to generate enough antigen specific Th1 type CD4 T cells.  There is a surprise in HIV pathogenesis considering a very few HIV specific CD4 T cells in the body.  In this case, SIV specific CD4 T cells.  During SIV infection, there is a sudden burst (4 hours after infection) of mRNA expressions of IL-17 and IFN-g in small intestine ileal loops.  This unexpected data suggest that certain Th1-like cells and Th17 cells are preexist in GI tract mucosal area.  Those IFN-g producing Th1-like cells may not be the typical antigen specific Th1 cells.  Production of IL-17 and IFN-g by these cells cannot be detected in SIV infected animals (Rhesus macaque), suggesting that CCR5 expressing CD4 T cells (HST: CD4 T cells susceptible to R5-tropic HIV) could be the source for the IFN-g producing cells.
Simian immunodeficiency virus-induced mucosal interleukin-17 deficiency promotes Salmonella dissemination from the gut.  Raffatellu M, Santos RL, Verhoeven DE, George MD, Wilson RP, Winter SE, Godinez I, Sankaran S, Paixao TA, Gordon MA, Kolls JK, Dandekar S, Bäumler AJ.
Nat Med. 2008 Apr;14(4):421-8. Epub 2008 Mar 23.).

Current paradigms on the generation of effector CD4 T cells
Current paradigms on the generation of effector CD4 T cells are rather simple and straightforward, although it takes more than two decades to establish them since the first use of terms, type 1 and type 2 CD4 T helper cells, Th1 and Th2, in 1986.  It is assumed that all effector CD4 T cells are derived from the naive T cells, directly exported from the thymus after T cell development and moved to lymph nodes, where the majority of naive T cells are found.  Naive T cells are defined as T cells that have never been exposed to an antigen and, therefore, never been activated.  They remain at a quiescent stage with minimal cell division.  To become an effector T cells, there is a special requirement.  Once naive T cells are activated, it becomes effector T cells and performs a variety of effector function as described above.

I challenge here that certain CD4 T cells with effector function, therefore considered effector T helper cells, could be derived from HST, but not from naive T cells.

Several effector T cells could be directly generated from HST (CCR5+ CD4 T cells).  That has been the central theme of my hypothesis.  IFN-g producing CD4 Th1 cells are generated in two waves after infection.  It is expected that early IFN-g producing cells have been considered as effector memory CD4 T cell populations that were generated even before pathogenic challenge.  Late arising Th1 cells are expected to be the newly formed Th1 cells from the pathogenic challenge derived from naive T cells.  However, these two types of Th1 cells are not only different from their timing of action, but they do not share requirement for their generation.  Early IFN-g producing Th1 cells, unlike conventional Th1 cells, do not even require IL-12'IL-12R/STAT4 axis for their differentiation from naive T cells.  Instead, they use IFN-g/IFN-gR/STAT1 axis. These differences suggest that they are different type of Th1 cells.  I called the early IFN-g producer Th1-like CD4 T cells, instead of Th1 CD4 T cells.  When infected with SIV in non-human primates, early IFN-g producing CD4 T cells (Th1-like) cells are missing, suggesting they could be derived from the HST, but not from naive T cells.  We will be able to answer this question when we were able to differentiate Th1 and Th1-like cells.  It is also expected that early Th1-like cells could be involved in a broad range of protective immune responses, unlike antigen specific Th1 cells.  They could be involved in innate immune responses, in addition to initiating adaptive immune responses by activating antigen presenting cells through CD40L-CD40 interaction (indirect hint by CD40L+ CD4+ memory T cells migrate in a CD62P-dependent fashion into reactive lymph nodes and license dendritic cells for T cell priming. Martín-Fontecha A, Baumjohann D, Guarda G, Reboldi A, Hons M, Lanzavecchia A,Sallusto F.
J Exp Med. 2008 Oct 27;205(11):2561-74. Epub 2008 Oct 6.).

The lack of HST after being deleted by HIV will affect the generation of effector T cells that are derived from them such as, Th1-like, Th2-like, TFH, Th17 cells and gut LP Treg cells.  This is different from our current paradigm.

Key words
effector CD4 T cells, HIV, AIDS, Th1, Th2, Th17, TFH, Treg cells, Th1-like, Th2-like, extrafollicular TFH, isotype switch, affinity maturation, HST, BnAb, Siliciano, Chase, Dandekar, Sekaly,

Why does the number of CD4 T cells decrease gradually in AIDS patients?

The number of CD4 T cells in HIV infected individuals decreases gradually, which takes place spanning a decade so, or even longer.  Why will that be the case?  How does it work?  Are all CD4 T cells supposed to be killed after activation by HIV as generally considered currently?  Are any CD4 T cells getting killed after activation by HIV?  These questions can easily be asked by any seasoned immunologist, since we do not have any general information on this issue, other than CD4 T cells are killed after activation by HIV.  Even though AIDS has been around almost three decades and we have known that HIV is a causative viral agent, this apparent phenomenon is not clearly explained and even not clearly understood to investigators of HIV pathogenesis.  Here, the possible mechanism that can explain this apparent phenomenon will be discussed.  
One of the earlier proposals to explain the gradual decline of total number of CD4 T cells was the "bathtub theory", one or two decades ago.  Bathtub theory states that CD4 T cells are repeatedly attacked and killed off by HIV without supplying and replenish them fast enough from the thymus. Therefore, the total number of CD4 T cells eventually drained lie the water level in a bathtub with slower water supply but a full drain.  This hypothesis may have been proposed even before the significance of R5 tropic virus depleting all CD4 T cells found in mucosal effector sites.  Other than bathtub theory, there is not many alternative explanations to explain the gradual decline of total CD4 T cells, an important indicator of the progression of AIDS.
This is one of the hypothesis that I come up with why it takes so long to kill off all the CD4 T cells, if HIV infects and kills only activated CD4 T cells.


Highlights of the theory
The bottom line of this theory is that HST (CD4 T cells that are susceptible to R5-tropic HIV) are all killed within a few weeks by R5-tropic HIV after infection.  None of the non-HST are direct targets for R5-tropic HIV, unless they express CCR5.  However, there is a possibility that R5 tropic HIV can gain tropism for CXCR4 and become an R5/X4 dual tropic HIV.  In addition, not only R5-tropic HIV, but also X4 tropic HIV is also co-infected an individual.  I have to emphasize that infection of X4 tropic HIV will not cause AIDS.  

• HST are infected by the R5-tropic HIV and depleted rapidly.
• The loss of HST will lead to the failure in a maturation of antigen presenting cells (APCs), such as dendritic cells.
• The lack of mature APCs fail to activate naive CD4 T cells, which were developed in the thymus and migrated to the lymphoid organs.
• Since naive T cells cannot survive forever and have a life span as a quiescent stage, the number of these population will gradually decrease, leading to the decline in total number of CD4 T cells.
• The proposal suggests a three step process for the gradual decrease in a total number of CD4 T cells.  
1. the loss of HST
2. Failure to activate antigen presenting cells due to the loss of HST
3. Failure to activate naive T cells within their life-span

Studies supporting above hypothesis:  (Many of them are for professionals only)

• HST (CCR5+ CD4 T cells) are depleted within a matter of weeks (by many reviews written by many authors, such as Picker L, Douek D, Silvestri G etc.).  This is a well established phenomenon.
• Effector memory T cells (TEM) are professional CD40 ligand expressing CD4 T cells (by Sallusto's group in JEM).  HST share the majority of phenotypes with TEM. 
• APCs becomes fully matured by interactions of CD40L-CD40.  This is well established among immunologists for two decades.
• Several recent studies and concept of naive T cells suggest that naive T cells are in the stage of quiescence, just like many stem cells.  They are live, but in a stage of quiescence, just surviving without much expenditure of energy.
• Certain molecules are involved in maintaining quiescence.  Those are, Foxo proteins, transcription factors KLF2, Id2, etc.  
• The lack of these proteins in mouse lacking genes encoding these proteins usually lose naive T cell population, without leaving behind effector memory T cells at the mucosal effector sites.
• Dipeptidyl Peptidase 2 (DPP2), by Dr. B. Huber of Tufts, has been linked to the survival of quiescent cells. Partial deficiency of DPP2 by knockdown expression of a gene encoding DPP2 resulted in the generation of Th17 cells, suggesting Th17 cells are not derived from naive T cells.  This hypothesis needs several leaps of imagination.

Key words:

HIV, AIDS, CD4 T cells, CCR5 tropic, CXCR4 tropic, HST (CD4 T cells that are susceptible to R5-tropic HIV infection), antigen presenting cells, naive T cells, effector memory T cells, Picker, Douek, Silvestri, Sallusto, Huber, Th17, quiescent, stem cells, FOXO, KLF2, Id2, 

Adaptive Immune Responses against HIV

There are two different types of adaptive immune responses against virus.  The word "adaptive" is used to counter against the word "innate".  Adaptive immune responses involve cell mediated immune responses and antibody mediated immune responses.  Cell mediated immune responses are delivered by T cell, while antibody-mediated immune response, also called humoral immune response, by antibody produced by B cells.
It is generally regarded that viral infection is cleared by both cell mediated and antibody mediated immune responses. Cell mediated immunity (CMI) involves killing virus infected cells by cytotoxic T cells (CTL, or CD8 T cells) that are specific to a virus derived peptide presented by an MHC class I molecule.  Since virus-peptide specific cytotoxic T cells are very rare, it takes several days to weeks to generate enough CTLs to fight against virus specific T cells.  However, once enough CTLs were generated and fight off the virus-infected cells, the majority of them dies and a small number of cells are saved for the future use. Those saved are called memory CTL.  They are quick to respond to the second encounter of the same virus infected cells and that is the basis for the T cell vaccine against a specific virus.
Antibody mediated immune (AMI) response is delivered by antibodies that were generated against the cell surface (exposed) antigen.  To induce AMI by vaccination, it is critical to generate a specific cell surface molecule (called epitope) that is ready to elicit antibody against it.  Soluble antibody is generated by mature B cells (or plasma cells; secreted antibody producing cells).  Once generated, antibody recognizes certain part of the virus and antibody coated virus becomes readily taken care of by several mechanisms, including the opsonization.  Unlike cell mediated immune responses, antibody directly recognizes virus outside of the cells.
In most cases, it is either CMI or AMI protects hosts against a variety of infections of pathogenic viruses.  However, innate immune responses always try to take care of them in the first place.  It is not clear which one of the two adaptive immune responses, or both, were actually involved in protections against a specific viral infection.  It is not apparent whether our immune system chose one or another if they failed to take care of a pathogen in the first place.  Who will decide which one of the two adaptive immune system, CMI or AMI, to use?

CD4 T cells are essential components for CMI and AMI in several various steps.  It requires several books to cover the detailed information on the role of CD4 T cells in immunity .

Scientists have been struggling for the last two to three decades to generate a vaccine against HIV without any noticeable progress.  Initially, it was against cell surface molecules, env, to generate antibodies against it.  It did not work out.  I am not so sure what were the criteria for evaluating vaccine efficacy.  They switched back to a vaccine strategy which is geared towards generating CTLs against intracellular proteins, gag, pol and nef.  Big scale trials, called STEP, resulted in an increase in HIV infection in vaccinated individuals.  It was haulted prematurely for an apparent reason, increase in HIV susceptibility after immunization.  Recent story, called RV144, evolves around antibody mediated immune responses, even though it is still under investigation.  It showed positive data for the first time.  51 people out of 8000 vaccinated were infected with HIV, while 74 were infected with HIV among the similar number of unvaccinated control group.  They claimed that it is very significant statistically, 30% of vaccine efficiency, even though it seems not a thrilling number.  Currently, active study is ongoing to understand what made RV144 was marginally successful, especially to understand which immune correlates are different between uninfected and infected individuals among participants of RV144. The progress has been published recently in Nature Immunology (NATURE IMMUNOLOGY VOLUME 13 NUMBER 5 MAY 2012).
http://www.nature.com/ni/journal/v13/n5/full/ni.2264.html

I will summarize the RV144, in a separate article.
Recent meeting "CROI 2012" has a very intense discussion on RV144.  Dr. Richard Koup of VRC summarized the current status on HIV vaccine in his presentation with a sum of RV144.
http://retroconference.org/static/webcasts/2012/

Issues needed further consideration:

• It seems that the pendulum for the HIV vaccine is leaning towards the generating broad neutralizing antibodies (BnAb) at the moment.  
• High affinity antibodies will require a lot of help from the follicular helper T cells (TFH), in several steps of their development.
• It will be interesting to see whether neutralizing antibody against HIV (SIV) is present (abundant) in lucky individuals with homozygous mutant CCR5 gene (CCR5delta32) or not.
• It will be interesting to see whether BnAB is generated from non-pathogenic infection of SIV into non-human primates.
• If they do have high titer of anti-HIV (SIV) antibodies, it suggests that the lack of neutralizing antibodies are not necessarily due to an innate difficulty of generating antibody against HIV, but the lack of functional HST causes the inability to generate neutralizing antibodies against HV.
• It will suggest that the loss of HST is directly involved in the generation of BnAb.
• There is a strong possibility that TFH cells could be missing in HIV infected individuals, due to the lack of HST.
• It will suggest that our current paradigm that TFH is derived from naive T cells will require further examination.
• It is also applied to the generation of Th17 cells, which is lacking in HIV infected individuals with many naive T cells.

Key words:

AIDS, HIV, vaccine, HST, Adaptive Immune responses, innate immune responses, B cell, T cells, humoral immunity, cell mediated immunity (CMI), antibody mediated immunity (AMI), Humoral immune response, STEP, RV144, CCR5, CCR5delta32, env, gag, pol, nef, CTL (cytotoxic T cell), CD8 T cell, TFH, Th17, naive T cell, CROI, Richard Koup, VRC (Vaccine research center) 

CCR5 (R5)-tropic HIV Susceptible CD4 T cells (HST).

What is it?
R5-tropic HIV Susceptible CD4 T cells (HST).
CD4 T cells that are susceptible to an infection by R5 (CCR5)-tropic HIV.

Why are they so important?
Because the lack of HST undoubtedly leads to AIDS and providing the HST with a homozygous mutant form of CCR5 (CCR5delta32) cured an AIDS patient by reconstituting an immune system.
Moreover, individuals who have homozygous mutant form of HST (CCR5delta32) are resistant to getting AIDS even in the presence of high titers of R5-tropic HIV.

Where do they reside in the body?
The majority of CD4 T lymphocytes is enriched in the lymphoid organs, lymph nodes and spleen after being generated/developed in the thymus by the positive and negative selection via MHC class II molecules.  However, the HST are enriched in non-lymphoid tissue.  This could be due to the fact that HST do not express a chemokine receptor CCR7.  Therefore the phenotype of HST is CCR5+CCR7-.  In addition, they share many characteristics with effector memory T cells, such as CD45RBhigh, CD62Llo, etc.  This is another important and specific topic that will be dealt in another essay.  It is also noteworthy that the number and the composition of CD4 T cells in the mucosal effector sites, such as gut, skin and lung, seem to be controlled by commensal microbiota, instead of an antigen presented by MHC class II molecules.  This also provides a clue that HST could be a different type of CD4 T cells, distinct from conventional CD4 T cells found in lymphoid organs.

How does HST work?  (A billion dollar question that has never been asked)
This question can be rephrased as "Why does the loss of the HST lead to AIDS"?  It needs a lot of fundamental understanding of HST before we try to develop a vaccine against HIV and AIDS.

This is my current theory, I mean, the THEORY.

1. HST is not derived from thymus (extrathymic T cells).
2. HST comes in two different forms, one with FoxP3 and the other without FoxP3 (FoxP3+ and FoxP3-).
3. HST can become a variety of effector T cells, such as Th17, Th1-like, Th2-like and TFH.
4. HST are activator for antigen presenting cells (that can induce maturation of APCs via CD40L-CD40 interaction).
5. HST respond directly to the pathogen associated molecular pattern (PAMP) via pattern recognition receptor (PRR).
6. The number and the proportion of HST in mucosal effector sites are governed by the commensal microbiota, instead of the presence or absence of peptide antigen presented by MHC class II molecules.

So, put them altogether,
HST are CD4 T cells mainly located in the gut mucosal effector sites.  Their main job at the mucosal effector sites is to make sure that there is no aberrant and unnecessary immune responses by suppressing immune responses (peace keeper, police, during non-pathologic condition).  They are doing this through the IL-10, TGF-b and FoxP3 mediated immune suppression. During infection, they can rapidly change their phenotypes and turn into a variety of effector T cells (it is called plasticity of T helper cell).  In other words, they are sentinels at the site where immune system directly contacting incoming pathogens.  Depending on the situation (pathogens encountered), once peace-keeping HST can become one or many of the effector T cells (in other words, police in action for protecting host until soldiers are involved).  They can become Th1- like, Th-2 like, Th17, or TFH cells, all depending on the nature of the pathogens.  They probably differentiate fast at the mucosal effector sites, not at the lymphoid organs.  Unlike the majority of CD4 T cells found in lymphoid organs, they react directly to the PAMP through the PRR expressed on HST, suggesting their expression of PRR, such as TLR or NLR.  Unlike CD4 T cells in lymphoid organs, the number and frequency of HST are governed by the presence and nature of commensal and incoming microbiota.  All in all, they seem to be totally different from CD4 T cells found in lymphoid organs.  This is the type of cells that will be missing when R5-tropic HIV infects and destroys them all.  Some cytokines and cytokine receptors will differentially express on HST.  IL-2, IL-15, IL-27 and their receptors could be different ones.

What will be the immunological outcome of lacking HST after being completely destroyed within a matter of weeks by R5-tropic HIV?  
In the absence of HST, incoming pathogens will get into our body without any resistance.  In addition, the growth of commensal microbiota will not be controlled and once beneficial commensals become harmful due to their overcrowding.  It is easy to imagine that the lining of the gut starts to leak due to an imbalance triggered by overgrowth of microorganisms, which is a common occurrence to the AIDS patients (leaky guts and microbial translocation).  When there is an imbalance in mucosal environment in healthy individuals, HST readily becomes Th17 cells and other effector T cells.  IL-17 produced by Th17 cells recruit neutrophils to the area and they start to kill (phagocytose) and control the unchecked growth of bacteria and other microbial organisms.  In addition, IL-22 produced by Th17 cells repair epithelial linings disrupted by outgrowing microorganisms.  The loss of Th17 cells is one of the characteristics of an AIDS patient.  HST can turn into the interferon-gamma (IFN-g) producing Th1-like or IL-4 producing Th2-like effector T cells without major differentiation process, which usually occurs in the lymphoid organs from naive T cells with a specific peptide antigen.  Conventional T cell differentiation will require several complex and time-consuming steps that involve antigen processing, naive T cell activation, proliferation and differentiation, which usually take more than a week.  Proliferation, and differentiation of HST could be a lot quicker and immediate to counter incoming pathogens, which are usually multiplying every hour or so.  Moreover, data are accumulating, which suggesting two different types of Th1 andTh2 cells. It is highly likely that generation of broad specificity neutralizing antibodies (bnAb) against HIV will require TFH, which could be derived from HST.  Should that be the case, current hopes of HIV vaccine trials to boost antibody mediated anti-HIV immunity will need careful evaluation of each step.  I will expand this part in another essay.

Can we protect HST from R5-tropic HIV infection and destruction?
Yes, theoretically in two different ways.  First, by blocking one of the two molecules CD4 and CCR5, from adhering by HIV.  Many investigations have been tried including decoy receptors and soluble form of these molecules and antagonist for these molecules.  They are all valuable methods to try, without much success yet.  But they are not vaccine approaches and will not educate our immune system.  It is a very challenging task.  The more you know about the underlying mechanism, it becomes much more complicated.  Sometimes, our current paradigm of immunology hinders the progress required to understand HIV pathogenesis.  The HIV did it to survive in our hostile environment.  Can we counter them?  I hope so, much sooner than later.  We still need to understand how they are doing it, if we can counter them appropriately.  It seems like a long process and will definitely require ingenious thoughts derived from "outside the box".

Key words
HST, R5-tropic, HIV, AIDS, CCR5delta32, CCR7, effector memory T cells, CD45RB, CD62L, commensal microbiota, mucosal effector sites, lymphoid organs, non-lymphoid organs, extrathymus, FoxP3, effector T cells, Th17, Th1, Th2 TFH, antigen presenting cells, pathogen associated molecular pattern (PAMP),  pattern recognition receptor (PRR), TLR, NLR (Nod-like receptor), interferon-gamma (IFN-g), IL-4, naive T cells, broad specificity neutralizing antibodies (bnAb), decoy receptor, antagonist

** In May 2012 Masopust's group reported that effector memory T cells resident in non-lymphoid tissue T(RM) are different from effector memory T cells (TEM) in lymphoid tissue.  This finding is 180 degree different from their original claim for a long period of time (for 10 years).  I hope that this is the beginning of recognizing HST as a new type of T cells different from non-HST.  Masopust and Lou Picker also wrote a review article on this issue called (dubbed Hidden Memory).  This is an exciting change for the development of HIV vaccine.

The followings were Copied from JI

J Immunol. 2012 May 15;188(10):4866-75. Epub 2012 Apr 13.

Antigen-independent differentiation and maintenance of effector-like resident memory T cells in tissues.

Casey KA, Fraser KA, Schenkel JM, Moran A, Abt MC, Beura LK, Lucas PJ, Artis D, Wherry EJ, Hogquist K, Vezys V, Masopust D.

Source

Abstract  Differentiation and maintenance of recirculating effector memory CD8 T cells (T(EM)) depends on prolonged cognate Ag stimulation. Whether similar pathways of differentiation exist for recently identified tissue-resident effector memory T cells (T(RM)), which contribute to rapid local protection upon pathogen re-exposure, is unknown. Memory CD8αβ(+) T cells within small intestine epithelium are well-characterized examples of T(RM), and they maintain a long-lived effector-like phenotype that is highly suggestive of persistent Ag stimulation. This study sought to define the sources and requirements for prolonged Ag stimulation in programming this differentiation state, including local stimulation via cognate or cross-reactive Ags derived from pathogens, microbial flora, or dietary proteins. Contrary to expectations, we found that prolonged cognate Ag stimulation was dispensable for intestinal T(RM) ontogeny. In fact, chronic antigenic stimulation skewed differentiation away from the canonical intestinal T cell phenotype. Resident memory signatures, CD69 and CD103, were expressed in many nonlymphoid tissues including intestine, stomach, kidney, reproductive tract, pancreas, brain, heart, and salivary gland and could be driven by cytokines. Moreover, TGF-β-driven CD103 expression was required for T(RM) maintenance within intestinal epithelium in vivo. Thus, induction and maintenance of long-lived effector-like intestinal T(RM) differed from classic models of T(EM) ontogeny and were programmed through a novel location-dependent pathway that was required for the persistence of local immunological memory.

Their original story

Cutting edge: gut microenvironment promotes differentiation of a unique memory CD8 T cell population.

Masopust D, Vezys V, Wherry EJ, Barber DL, Ahmed R.

J Immunol. 2006 Feb 15;176(4):2079-83.

Abstract

Whether tissue microenvironment influences memory CD8 T cell differentiation is unclear. We demonstrate that virus-specific intraepithelial lymphocytes in gut resemble neither central nor effector memory CD8 T cells isolated from spleen or blood. This unique phenotype arises in situ within the gut, suggesting that anatomic location plays an inductive role in the memory differentiation program. In support of this hypothesis, memory CD8 T cells changed phenotype upon change in location. After transfer and in vivo restimulation, gut or spleen memory cells proliferated, disseminated into spleen and gut, and adopted the memory T cell phenotype characteristic of their new environment. Our data suggests that anatomic location directly impacts the memory T cell differentiation program.

Review paper
Hidden memories: frontline memory T cells and early pathogen interception.

Masopust D, Picker LJ.

J Immunol. 2012 Jun 15;188(12):5811-7. Review.

Immunologic memory reflects the ability of a host to more effectively respond to a re-encounter with a particular pathogen than the first encounter, and when a vaccine mimics the first encounter, comprises the basis of vaccine efficacy. For T cells, memory is often equated with the anamnestic response, the ability of secondary lymphoid tissue-based (central) memory T cells to respond to pathogen exposure with a more rapid and higher magnitude production and infection-site delivery of pathogen-specific effector cells than observed in naive hosts. However, increasing evidence supports a fundamentally different kind of T cell memory in which differentiated, long-lived effector memory T cells, prepositioned in sites of potential pathogen invasion or rapidly mobilized to such sites from blood and marginated pools, intercept and potentially control/eliminate pathogen within hours of infection. In this article, we review the evidence for this "hidden" T cell memory and its implication for vaccine development.

Do all CD4 T cells become targets for HIV infection after activation?

Do all CD4 T cells become targets for HIV infection?

This is a simple but important question that can easily be answered and the outcome would benefit tremendously to understand the HIV pathogenesis correctly.  If I ask this question to many current immunologists who have spent the majority of their professional lives on CD4 T cell, they would not be able to give you a correct answer, let alone from virologists working on HIV pathogenesis.  Therefore, I decided to describe our current consensus (understanding) on this issue and will add my own version to this.

Current views:

• Not all CD4 T cells are readily infected by HIV.
• CCR5 expressing CD4 T cells can only be infected by R5-tropic HIV.
• CXCR4 expressing CD4 T cells can only be infected by X4-tropic HIV.
• R5/X4-tropic HIV can infect CD4 T cells that express either CCR5 or CXCR4.

What is not clear:

• Do all CD4 T cells have potential to express CCR5 and/or CXCR4?
• Can a specific group of CD4 T cells express either CCR5 or CXCR4?
• What is the general tendency of expression of these molecules among all CD4 T cells?

My version:

• Only a specific group of CD4 T cells is able to express CCR5 (HST)
• The majority of CD4 T cells is able to express CXCR4, especially after activation.
• There is a possibility that CCR5 expressing CD4 T cells (HST) can express CXCR4 before or after activation.  But this may not be an important issue.
• The loss of CXCR4 expressing CD4 T cells are not as critical as the loss of CCR5 expressing CD4 T cells.  (Evidence: Berlin patient)

Significance:

• Infection of CD4 T cells by X4-tropic HIV infection may not be as critical as that of R5-tropic HIV infection, since the loss of CCR5 expressing CD4 T cells undoubtedly leads to AIDS.
• The loss of a small population of activated CD4 T cells by X4-tropic HIV infection would leave a small hole in our protective immunity.  Fox example, the loss of certain flu-peptide specific CD4 T cells is not life threatening or leads to systemic immunodeficiency as the loss of HST (HST: CD4 T cells that are susceptible to R5-tropic HIV).  
• But that is not the case, when we are infected with R5-tropic HIV.  They wipe off all CCR5 expressing CD4 T cells and undoubtedly will lead to an onset of AIDS.
• Without the presence of R5-tropic HIV, there may not be AIDS.
• Therefore, it could be very important to be able to separate R5-tropic vs. X4-tropic HIV infection.
• Why does the loss of CCR5 expressing CD4 T cells lead to AIDS?
• Why does the loss of total CD4 T cells happen gradually, while the HST are depleted within a matter of weeks after R5-tropic HIV infection?
• The above questions are apparent ones that are in need of immediate answers.

Relevant publication:

The following is a very specialized issue for advanced immunologists:  It will probably get you bored from the get-go.

Low levels of SIV infection in sooty mangabey central memory CD4⁺ T cells are associated with limited CCR5 expression

Paiardini, Silvestri and their colleagues from around the world (published in the Nature medicine).  

http://www.nature.com/nm/journal/v17/n7/full/nm.2395.html (Nature Medicine 2011). Their abstract is as follows,

"Naturally simian immunodeficiency virus (SIV)-infected sooty mangabeys do not progress to AIDS despite high-level virus replication. We previously showed that the fraction of CD4⁺CCR5⁺ T cells is lower in sooty mangabeys compared to humans and macaques. Here we found that, after in vitro stimulation, sooty mangabey CD4⁺ T cells fail to upregulate CCR5 and that this phenomenon is more pronounced in CD4⁺ central memory T cells (T_CM cells). CD4⁺ T cell activation was similarly uncoupled from CCR5 expression in sooty mangabeys in vivo during acute SIV infection and the homeostatic proliferation that follows antibody-mediated CD4⁺ T cell depletion. Sooty mangabey CD4⁺ T_CM cells that express low amounts of CCR5 showed reduced susceptibility to SIV infection both in vivo and in vitro when compared to CD4⁺ T_CM cells of rhesus macaques. These data suggest that low CCR5 expression on sooty mangabey CD4⁺ T cells favors the preservation of CD4⁺ T cell homeostasis and promotes an AIDS-free status by protecting CD4⁺ T_CM cells from direct virus infection."

My personal view on this paper: This is an excellent paper on one of the most important issues in HIV and AIDS research.  Why sooty mangabey is not getting an AIDS-like syndrome even with high titers of SIV?  As usual, they did a very good job.  One of the issues that I do not totally agree with them are marked as a bold/underlined above.  Their experiment is based on an assumption that CCR5 is expressed on the 'so called' effector memory CD4 T cells (T_EM), and they are derived from the central memory T cells (CD4⁺ T_CM cells).  That is our current common understanding and paradigm, especially to several investigators for HIV pathogenesis.  However, there is a strong possibility that there is a separate lineage of CD4 T cells that are exclusively able to express CCR5 molecules.  Instead of upregulation as they have claimed, a small number of those cells could be differentially expanded.  I will leave detailed discussion to the current professionals.  Nonetheless, it is one of the essential and basic knowledge to be resolved to design an effective vaccine against HIV.

Key words:
CD4 T cell, HIV, immunologist, virologist, CCR5, CXCR4, R5-tropic, X4-tropic, AIDS, HST, Paiardini, Silvestri, Nature Medicine, SIV, sooty magabey, central memory T cells, effector memory T cells, homeostatic proliferation, rhesus macaque

Is it HIV or an Immune System which causes an Onset of AIDS?

Highlights

• HIV triggers pathogenesis that leads to an onset of AIDS (Therefore, answer for the above question is yes, it is HIV)
• Fast mutation rates of HIV cannot be used always to explain the lack of protective immunity against them leading to AIDS, since the identical HIV cannot cause an onset of AIDS in certain individuals.
• An individual with a homozygous mutation of CCR5 (CCR5delta³² ) is not susceptible to AIDS even with high titer of HIV in an individual (no, it is not HIV)
• When HST is infected and depleted by HIV, it will lead to an onset of AIDS. However, as long as HST is spared by a depletion mediated by HIV or reconstituted by HST with a homozygous mutant form (CCR5^D32 ), there will be no AIDS (again, it is an immune system which plays a critical role of getting AIDS, especially of an availability of HST).
• But it all starts from HIV infection (yes, it is HIV)

The simple question posed above is a very important one and the correct answer will undoubtedly help us to design an effective vaccine against the HIV.  There is an apparent shortcoming in our understanding on a pathology triggered by an HIV infection, otherwise we should have had an HIV vaccine probably 20 years ago as initially claimed after identifying HIV as an AIDS causing virus.  At that time and still now, it may not be an unrealistic claim, since we routinely generate seasonal flu vaccine within a very short period of time, within a few months or so after identifying the disease-causing flu virus.  Apparently, it has not been easy for generating an HIV vaccine even after two long decades with extensive and expensive efforts. To identify what our shortcomings are and why it is so different from other viruses,  let's think about an apparent question first.

Does an HIV infection cause an onset of AIDS?  
The simple answer for this question is, yes and no.  It is yes, because without HIV infection, there is no onset of AIDS.  However it is no, because certain individual with high titers of HIV still does not suffer from AIDS.  An infection of the same rapidly-mutating and deadly HIV is no different from that of influenza virus for those lucky individuals.  Why that is the case?  Does HIV escape immune responses by their fast mutation rate?  If that is the reason, it is not easy to explain this discrepancy.  It is safe to say that HIV triggers initial punch leading to an AIDS.  However, there is a certain immunological basis that can explain why an HIV infection, unlike any other pathogens, causes an onset of AIDS.  Once we understand the immunological basis that was initially triggered by an HIV infection, we will be able to counteract correctly.  Don't we understand even after close to three decades of identification of HIV as an agent for AIDS?  I am afraid to say that we are not.  We do not know how to counteract with them.  But we repeat ourselves from old technologies and empirical lessons.  Waiting for miracles and hold onto tight to a small possible success, only can be visible by minute statistical differences.  

What kind of immune response make them HIV infection so dangerous and different from other infectious agents?
In one word, it is the type of target cells that HIV infects and destroys.  Unlike any other infectious agents, HIV, especially R5-tropic HIV (HIV that uses CCR5 and CD4 molecules as their target for initial attachment to infect cells) infects, expands initially and destroys them all at the mucosal effector sites, such as gut, lung and reproductive organs.  Once HIV is expanded, they hide as a latent form and sometimes change their phenotypes with their fast mutation rates.  I called those CCR5 expressing CD4 T cells as HST, HIV Susceptible CD4 T cells, meaning CD4 T cells that are susceptible to R5-tropic HIV.  Once HST is depleted with a fast and furious pace, there is no looking back. The titer of HIV is high in the system and HIV is waiting for available target cells for their propagation.  Even with Highly Active Antiretroviral Therapy (HAART) cannot rescue HST once gone, while plasma level of HIV can easily be controlled by it.  However, there is a possibility that newly formed HST can be protected by the lack of available HIV in the plasma by HAART.  That could be the reason that HAART needs to be very quick once infection occur and it require lifetime long treatment once infected by HIV.

Lessons learned from studies with non-human primates
Another strong case, which supports the the lack of immune system causes AIDS is derived from studies of non-human primates infected with SIV (human homolog of HIV).  Certain native species of non-human primates are not susceptible to AIDS-like syndrome even after SIV infection.  Although it is very difficult to test whether all SIV are identical or due to differences in virulence factor among different SIV species, it suggests that it is not all viruses that causes AIDS-like syndrome in non-human primates, but differences in preserving HST.  Some preserve HST by the lack of of expression of CCR5 (Sooty mangabey), while others express less CD4 molecules (African green monkey).  Clearly saving HST is the prime significance for not getting AIDS.  In all experimental infection of SIV study, Rhesus macaque (Chinese monkey) is used as a control, since they are prone to AIDS while losing the majority of CCR5 expressing CD4 T cells in a very fast pace.

Conclusion
AIDS is triggered by an infection of HIV.  Without HIV, there will be no AIDS for sure.  However,  AIDS won't happen if HST is spared even in the presence of HIV infection.  Therefore, it is for sure that HIV is the main agent that infects and depletes the HST.  
The answer to the original question is yes, it is HIV and yes it is the lack of immune system, in particular HST, which are destroyed by the presence of R5-tropic HIV.

Key words
Epitope, HIV, AIDS, vaccines, CCR5, HST, delta32 CCR5 homozygous, HAART, African green monkey, Sooty mangabey, Rhesus macaque,