We did not observe Csf1 expression in CD4+ T cells cultured ex vivo with either Th1- or Th2-polarizing cytokines (Fig 3G), even upon restimulation (S4 Fig), suggesting that additional signals provided in the context of in vivo infection are required to upregulate Csf1. Robust induction of the canonical cytokines Ifng and Il4 in these Th1 and Th2 cultures, respectively, served as a control to confirm effective polarization (Fig 3G). Here, IL4 is linked to infection.