CD4 and infection: Based on the experience that CD8 T cells are dependent on CD4 T cells to help with the resolution of chronic LCMV infections (Matloubian et al, 1994; Kalams & Walker, 1998; Aubert et al, 2011), we transferred CD3+ T cells from LCMV‐immune WT mice (> day 100 after infection) containing virus‐specific memory T cells (Fig EV2A) into diseased Jinx mice on day 15 and followed HLH progression.