Cytokine Diversity in the Th1-Dominated Human Anti-Influenza Response Caused by Variable Cytokine Expression by Th1 Cells, and a Minor Population of Uncommitted IL-2+IFN[gamma]- Thpp Cells

Within overall Th1-like human memory T cell responses, individual T cells may express only some of the characteristic Th1 cytokines when reactivated. In the Th1-oriented memory response to influenza, we have tested the contributions of two potential mechanisms for this diversity: variable expression of cytokines by a uniform population during activation, or different stable subsets that consistently expressed subsets of the Th1 cytokine pattern. To test for short-term variability, in vitro-stimulated influenza-specific human memory CD4+ T cells were sorted according to IL-2 and IFN[gamma] expression, cultured briefly in vitro, and cytokine patterns measured after restimulation. Cells that were initially IFN[gamma]+ and either IL-2+ or IL-2- converged rapidly, containing similar proportions of IL-2-IFN[gamma]+ and IL-2+IFN[gamma]+ cells after culture and restimulation. Both phenotypes expressed Tbet, and similar patterns of mRNA. Thus variability of IL-2 expression in IFN[gamma]+ cells appeared to be regulated more by short-term variability than by stable differentiated subsets. In contrast, heterogeneous expression of IFN[gamma] in IL-2+ influenza-specific T cells appeared to be due partly to stable T cell subsets. After sorting, culture and restimulation, influenza-specific IL-2+IFN[gamma]- and IL-2+IFN[gamma]+ cells maintained significantly biased ratios of IFN[gamma]+ and IFN[gamma]- cells. IL-2+IFN[gamma]- cells included both Tbet.sup.lo and Tbet.sup.hi cells, and showed more mRNA expression differences with either of the IFN[gamma]+ populations. To test whether IL-2+IFN[gamma]-Tbet.sup.lo cells were Thpp cells (primed but uncommitted memory cells, predominant in responses to protein vaccines), influenza-specific IL-2+IFN[gamma]- and IL-2+IFN[gamma]+ T cells were sorted and cultured in Th1- or Th2-generating conditions. Both cell types yielded IFN[gamma]-secreting cells in Th1 conditions, but only IL-2+IFN[gamma]- cells were able to differentiate into IL-4-producing cells. Thus expression of IL-2 in the anti-influenza response may be regulated mainly by short term variability, whereas different T cell subsets, Th1 and Thpp, may contribute to variability in IFN[gamma] expression.