T cell homeostasis in HIV infection: part of the solution or part of the problem?

Article Abstract:

Levels of CD4 T cells and CD8 T cells appear to be regulated by a non-selective mechanism. Both of these cells play a critical role in the body's immune system. HIV infection is characterized by the depletion of CD4 T cells and an abnormal ratio of CD4 to CD8 cells. Recent studies suggest that the body does not distinguish between the loss of CD4 cells or CD8 cells, but rather sees the loss of either one as a reduction in CD3 T cells. In response, the body then produces enough CD4 and CD8 cells to raise the overall CD3 level back to normal. As long as the CD3 level were normal, the body would not generate more CD4 T cells. One potential way of countering T cell depletion in HIV-infected individuals then, would be to intentionally lower CD8 levels and thus stimulate both CD4 and CD8 production. However, the consequences of reducing the number of CD8 cells, which kill HIV-infected cells, have not been studied yet. Further research on this and other approaches to T cell regulation are needed.

Editorial

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T cell homeostasis: implications in HIV infection

Article Abstract:

T cell levels may be regulated by a non-selective mechanism that does not distinguish between loss of CD4 T cells and loss of CD8 T cells. Both CD4 and CD8 cells play a critical role in the body's immune system. HIV infection is characterized by the selective depletion of CD4 cells and, at least initially, by an abnormal ratio of CD4 to CD8 cells. In mice depleted of CD4 cells, overall T cell levels returned to normal after about four months. However, the CD8 level was 163% of that in normal mice and the CD4 level was less than 63% of that in normal mice. This finding is consistent with the hypothesis that the body sees the depletion of either CD4 or CD8 cells only as T cell loss and produces both kinds until the overall T cell count returns to normal. It also suggests that one way of countering T cell depletion early in HIV infection would be to selectively deplete CD8 cells. In stimulating T cell production, CD4 levels would then rise.

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Blind T-cell homeostasis in CD4-deficient mice

Article Abstract:

Mice lacking CD4+ T cells appear to maintain normal total T-cell numbers, lending support to the hypothesis that overall T-cell homeostasis is maintained without controlling the numbers of specific T-cell types. This hypothesis is called the blind homeostasis hypothesis (BHH), in reference to the proposed control mechanism being blind to the difference between CD4+ and CD8+ T cells. Researchers counted T-cells in the blood and spleens of 19 mice with a genetic defect that makes them deficient in CD4+ T cells, and 29 mice with normal CD4+ T-cell counts. Although the total number of T cells in both groups was the same, the CD4-deficient mice had significantly elevated CD8+ T-cell counts in both their blood and their spleens. T cells with neither CD4 nor CD8 markers were also found in higher amounts in the CD4-deficient mice. The findings may help explain the mechanism of CD4+ T-cell loss that occurs during HIV infection.

Analysis, Development and progression, HIV infection, HIV infections, Cellular control mechanisms, Cell regulation, CD4 lymphocytes, CD8 lymphocytes

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