Aaron J. Tyznik

Interleukin-2 signals during priming are required for secondary expansion of CD8+ memory T cells.

Although interleukin-2 (IL-2) was initially characterized as the primary T-cell growth factor following in vitro activation, less is known about its role in shaping T-cell responses to acute infections in vivo. The use of IL-2- or IL-2-receptor-deficient mice is problematic owing to their early development of autoimmunity, attributable to the central role of IL-2 in the generation, maintenance and function of CD4+CD25+ regulatory T cells. To bypass these inherent difficulties, we have studied the effect of IL-2 on T-cell responses to acute infections by adopting a mixed chimaera strategy in which T cells lacking the high-affinity IL-2 receptor could be studied in an otherwise healthy mouse containing a full complement of regulatory T cells. Here we show that although IL-2 signalling to pathogen-specific CD8+ T cells affects the number of developing effector and memory cells very little, it is required for the generation of robust secondary responses. This is not due to an altered T-cell-receptor repertoire development or selection, and does not reflect an acute requirement for IL-2 during secondary activation and expansion. Rather, we demonstrate a previously unappreciated role for IL-2 during primary infection in programming the development of CD8+ memory T cells capable of full secondary expansion. These results have important implications for the development of vaccination or immunotherapeutic strategies aimed at boosting memory T-cell function.

Cutting Edge: The Mechanism of Invariant NKT Cell Responses to Viral Danger Signals

Invariant NK T (iNKT) cells influence the response to viral infections, although the mechanisms are poorly defined. In this study we show that these innate-like lymphocytes secrete IFN-γ upon culture with CpG oligodeoxynucleotide-stimulated dendritic cells (DCs) from mouse bone marrow. This requires TLR9 signaling and IL-12 secretion by the activated DCs, but it does not require CD1d expression. iNKT cells also produce IFN-γ in response to mouse CMV infection. Their mechanism of mouse CMV detection is quite similar to that of CpG, requiring both TLR9 signaling and IL-12 secretion, while the need for CD1d expression is relatively minor. Consequently, iNKT cells have the ability to respond to a variety of microbes, including viruses, in an Ag-independent manner, suggesting they may play a broad role in antipathogen defenses despite their limited TCR repertoire.

The CD8 Population in CD4-deficient Mice Is Heavily Contaminated with MHC Class II-restricted T Cells

In experiments to study the impact of deficiency in CD4+ T cell help on the magnitude of CD8+ cytotoxic T cell response to pathogens, it was noted that in CD4 gene knockout mice, the CD8 population made significant responses to several nominally major histocompatibility complex (MHC) class II–restricted epitopes in addition to the expected responses to MHC class I–restricted epitopes. A similar response by CD8+ T cells to class II–restricted epitopes was not observed in wild-type mice, or in mice that had been acutely depleted of CD4+ T cells just before the immunization. Coincident with this unexpected response to class II–restricted epitopes, it was also observed that the CD8+ response to the class I–restricted epitopes was consistently lower in CD4−/− mice than in wild-type mice. Further experiments suggested that these two observations are linked and that the CD8 population in CD4−/− mice may contain a majority of T cells that were actually selected by recognition of MHC class II molecules in the thymus. These results have implications for understanding CD4 versus CD8 lineage commitment in the thymus, and for the practical use of CD4−/− mice as models of helper deficiency.

The Surprising Kinetics of the T Cell Response to Live Antigenic Cells

Cooperation between CD4+ and CD8+ T cells is required for the proper development of primary effector and memory CD8+ T cells following immunization with noninflammatory immunogens. In this study, we characterized murine CD4+ and CD8+ T cell responses to male-specific minor histocompatibility (HY) Ags following injection of live male cells into females of the same strain. Male cells are rejected 10–12 days after transfer, coinciding with the expansion and effector function of CD8+ CTLs to two H-2Db-restricted epitopes. Although anti-HY CD4+ T cell responses are readily detectable day 5 posttransfer, CD8+ responses are undetectable until day 10. The early CD4+ response is not dependent on direct presentation of Ag by donor male cells, but depends on presentation of the male cells by recipient APC. The CD4+ T cell response is required for the priming of CD8+ T cell effector responses and rejection of HY-incompatible cells. Unexpectedly, HY-specific CD4+ T cells are also capable of efficiently lysing target cells in vivo. The delay in the CD8+ T cell response can be largely abrogated by depleting T cells from the male inoculum, and donor male CD8+ T cells in particular suppress host anti-HY CD8+ responses. These data demonstrate dramatic differences in host T cell responses to noninflammatory Ags compared with responses to pathogens. We explain the delayed CD8+ response by proposing that there is a balance between cross-presentation of Ag by helper cell-licensed dendritic cells, on the one hand, and veto suppression by live male lymphocytes on the other.