IL-7 and CD4⁺ T-cell proliferation

A recent article by Steffens et al1 assessed whether the IL-7 cytokine promotes productive HIV-1 infection in naive CD4⁺ T cells. This is an important biologic issue because, while most circulating peripheral T lymphocytes are in the G₀ state, the productive infection of both naive and memory CD4⁺ T cells with HIV-1, at least outside the context of lymphoid tissues,2 requires a progression into the G_1b stage of the cell cycle.3 It is assumed that, in vivo, HIV-1 infection occurs in CD4⁺ T cells that are cycling because they have recently been stimulated by their cognate antigen. One quandary that arises from this hypothesis is that those T cells stimulated through their T-cell receptor (TCR) attain characteristics of memory lymphocytes and lose their naive phenotype. The detection of HIV-1 in naive T cells in vivo, albeit at significantly lower levels than in the memory T-cell subset,4-7 raises questions regarding the mechanisms controlling HIV-1 infection in the former population. Thus it is possible that a cytokine such as IL-7, which does not significantly modify the naive phenotype of T cells,8-10 may be involved in this process.

A study of HIV-1 infection in the context of IL-7 stimulation requires an understanding of the biologic effects of IL-7. But while Steffens et al reported that naive human CD4⁺ T cells proliferated in response to IL-7 in vitro, our 2 groups have independently found that naive CD4⁺ T cells do not proliferate under these conditions.11,12 The recent data of Geginat et al13 support these observations; they find that in vitro IL-7 synergizes with IL-15 in driving the proliferation of memory CD4⁺ T cells but has no effect on naive T cells.13 Indeed, we determined that only recent thymic emigrants, naive CD4⁺ T cells isolated from umbilical cord blood, proliferate significantly in response to IL-7 in vitro.11,12 Moreover, even under conditions where T lymphocytes are cultured in the presence of both IL-2 and IL-7 cytokines, only recent thymic emigrants, and not naive CD4⁺T cells, enter into the S or G₂ phases of the cell cycle at significant levels (Figure 1). Finally, using a technique that permits simultaneous DNA and RNA quantitation, we previously demonstrated that, although 10% to 20% of IL-7–stimulated memory CD4⁺ T cells progress to the G_1b phase of the cell cycle, only 3% to 5% of naive CD4⁺ T cells progress past G_1a.12 

It should nevertheless be noted that all studies assessing the effects of recombinant IL-7 on human T-cell subsets, including our own, may be biased because they have been performed ex vivo. In mice, homeostatic T-cell proliferation is readily detectable, but only under conditions of lymphopenia. In a lymphopenic environment, proliferation of naive T cells in mice is conditional on TCR/selfpeptide/major histocompatibility complex (MHC)–derived signals14-22 but is also strictly dependent on the presence of endogenous IL-7.23,24 Proliferation of T cells in nonlymphopenic mice can be induced by injection of high doses of recombinant IL-7, resulting in cell-cycle entry of a low percentage of CD4⁺ T cells (approximately 4%) and a higher percentage of CD8⁺ T cells (10%-11%).25 Thus, in interpreting the in vitro studies performed with human T cells, it is important to take into account the limitations resulting from the absence of in vivo factors that likely cooperate with IL-7 in driving cell-cycle progression in lymphoid organs.

One technical difference that may explain the apparent discrepancies between the respective in vitro human T-cell studies performed by our groups11,12 and that performed by Steffens et al concerns the manner in which CD4⁺ T cells were selected. In order to eliminate confounding effects due to in vivo activated T cells, we removed T lymphocytes expressing the HLA-DR activation marker with a monoclonal antibody. HLA-DR⁺ cells account for 2% to 10% of the T-cell population, depending on the donor. Under conditions where these cells are removed, purified CD4⁺ T cells do not proliferate in response to either IL-7, the phytohemagglutinin (PHA) lectin, or a mitogenic anti-CD3 mAb.3,11,12These data are pertinent to HIV-1 infection because Korin and Zack found that, under conditions where CD4⁺ T cells are stimulated with an α-CD3 mAb, the presence of HLA-DR⁺cells modulates HIV infection: productive HIV infection is observed in the presence of HLA-DR⁺ cells, whereas in their absence, infection is blocked.3 Thus the HIV-1 infection observed by Steffens et al in IL-7–treated naive CD4⁺ T cells may be specific to their experimental conditions. Furthermore, Steffens et al found that only naive T cells from 4 of 6 donors were responsive to IL-7–mediated HIV infection, and as such it is possible that this discrepancy results from differences in the percentages of activated T cells present in these individuals at a given time. This hypothesis is attractive since IL-7 is not likely to exert its effect at the level of enhanced viral entry; while IL-7 up-regulated expression of the HIV gp120 coreceptor CXCR4, this phenomenon was also observed in T cells from donors who were resistant to IL-7–mediated infection.1 

In many of the experiments presented by Steffens et al, CD8⁺ T cells were maintained in the culture, and it is therefore possible that the capacity of naive CD4⁺ T cells to be infected by HIV-1 following IL-7 treatment is modulated by the presence of CD8⁺ T cells and/or activated T cells. Indeed, both in vitro and in vivo, CD8⁺ T cells appear to undergo significantly more division in response to IL-7.1,25Further studies will be necessary to distinguish the effects of IL-7 on naive CD4⁺ T cells in the context of distinct hematopoietic cell populations. Finally, it will be important to elucidate how the cellular environment in lymph nodes and the peripheral circulation differentially regulates the responsiveness of naive T cells to IL-7 and HIV-1 infection. Specifically, the in vivo data obtained in mice suggest that the effects of IL-7 may differ in conventional in vitro cultures and in the context of the 3-dimensional architecture and cellular environment present in lymphoid tissues.24 25 The data obtained from the ensemble of these experiments will allow us to determine whether, and under what conditions, recombinant IL-7 may be beneficial as an immune modulator in HIV-infected individuals.