Although the high frequency of IgM ASCs in DR1 mice after PR8 infection raised the possibility of an unusually strong T-independent Ab response to this virus [24], we found that the formation of both IgM and IgG ASCs specific for PR8 in the MedLN of DR1 mice on day 8 was largely eliminated by CD4 T cell depletion (Fig. S3), indicating a dependence on CD4 T cell help. Here, CD4 is linked to infection.