IL21 and viral infectious disease: While these findings support a direct role for IL-2/IL-21 signaling on CD8 T cell maintenance during chronic LCMV infection, they do not exclude a concomitant role of these IL-21-producing CD4 T cells for humoral immunity as IL-21 is a hallmark cytokine of follicular T helper cells (TFH), as discussed later in Section “Differentiation of CD4 T Cells During Active Chronic Viral Infections.”