Blood monocytes, myeloid dendritic cells and the cytokines interleukin (IL)‐7 and IL‐15 maintain human CD4+ T memory cells with mixed helper/regulatory function*

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Abstract
The number and function of human T cells in the periphery are regulated by homeostatic signals received from antigen-presenting cells (APCs) and the common gamma chain (γc) cytokines interleukin (IL)-7 and IL-15. We found that, in the absence of introduced antigen, blood monocytes or myeloid dendritic cells (MDCs) in the presence of IL-7 and IL-15 (IL-7/IL-15) can regulate CD4+ T memory (Tm) cell numbers by polyclonal cell proliferation. The dynamics of CD4+ Tm cell proliferation, in the presence of IL-7/IL-15, was dependent on contact with MDCs and to a lesser extent on contact with monocytes. IL-7/IL-15 either alone or combined with monocytes or MDCs enhanced the proportion of CD4+ Tm cells with activated and effector phenotype and diminished the helper function of CD4+ Tm cells. These CD4+ Tm cells, preconditioned with IL-7/IL-15 alone or with monocytes or MDCs and IL-7/IL-15, reduced T cell-dependent immunoglobulin M (IgM) and IgG responses. This appeared to be a contact-dependent effect involving a reduction in antibody-producing CD27+ B memory cells, but contact-independent suppression by soluble factors also contributed to the antibody-producing capacity of CD27+ B memory cells. These results indicate that blood monocytes, MDCs and the cytokines IL-7/IL-15 contribute to homeostasis of CD4+ Tm cells by regulating their number, activation state and helper/suppressor (regulatory) function. In healthy individuals, this mode of regulating CD4+ Tm cell homeostasis may provide a basis for the control of autoimmune responses.