R5-tropic HIV is the dangerous one since the loss of CCR5 expressing CD4 T cells (HST) by R5-tropic virus infection leads to AIDS. On the other hand, X4-tropic HIV is frequently found in a latent stage of HIV infected individuals in a variety of cell types including CD4 T cells and hematopoietic stem cells. Accumulated data suggest that an onset of AIDS will not occur without R5-tropic HIV infection since the loss of HST is the main cause of AIDS and sparing HST (or providing a mutant form of HST) will be able to cure AIDS. CD4 T cells, which were reconstituted by a bone marrow transplant in the Berlin patient have a potential to become CXCR4+ after activation and can readily be infected by X4-tropic, but not by R5-tropic HIV. The Berlin patient has a low titer of HIV and is free from AIDS symptoms, which suggests that infection by X4-tropic HIV can be taken care of by a host protective immune system and will not cause an onset of AIDS.
R5-tropic HIV
Originally called M-tropic virus, meaning that macrophages are the major target cells for R5-tropic HIV. I am not so certain whether it holds true until now. The significance of the R5-tropic HIV is that they infect CCR5 expressing CD4 T cells mainly found at the mucosal effector sites, such as gut, vagina and lung. The loss of those cells is the main culprit for HIV pathogenesis and directly associated with an onset of AIDS.
X4-tropic HIV
X4-tropic HIV seems to be easily be taken care of by a protective immune system without causing a lethal disease. The main target cells for the X4-tropic HIV could be activated CD4 T cells and remain as latent form when they are rested. It is also possible that X4-tropic HIV can become R5-tropic or R5-tropic HIV can become X4-tropic HIV by a notorious rapid mutation rate of retrovirus. It is a common occurrence that later stage of AIDS patient has a dual tropic (R5X4-tropic) HIV. Nevertheless, X4-tropic HIV may not be a main HIV that causes an onset of AIDS.
My personal thoughts: HST could be a different type of CD4 T cell that shares certain similarities to macrophages, such as those expressing several molecules expressed on macrophages. This important issue has never been noticed and deeply investigated. Since HST could be a totally different type of T cells with paramount significance in a protective immunity, protecting HST from R5-tropic HIV infection could be an ultimate goal of HIV vaccine.
If we were able to look at an outside of the box, we may have a new insight on HIV pathogenesis and AIDS.
Relevance to HIV vaccine development:
Since R5-tropic HIV is the major AIDS causing virus and they destroy HST rapidly, this concept has to be established in advance even before trying a variety of expensive and extensive HIV vaccine trials.
Key words:
CCR5, CXCR4, M tropic, T tropic, HST, HIV, AIDS, vaccine, CD4, macrophage,
http://soonhong-hivandaids.blogspot.com/2012/04/lacking-hst-leads-to-aids-and-providing.html
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