Infection order outweighs the role of CD4⁺ T cells in tertiary flavivirus exposure

The link between CD4⁺ T and B cells during immune responses to DENV and ZIKV and their roles in cross-protection during heterologous infection is an active area of research. Here we used CD4⁺ lymphocyte depletions to dissect the impact of cellular immunity on humoral responses during a tertiary flavivirus infection in macaques. We show that CD4⁺ depletion in DENV/ZIKV-primed animals followed by DENV resulted in dysregulated adaptive immune responses. We show a delay in DENV-specific IgM/IgG antibody titers and binding and neutralization in the DENV/ZIKV-primed CD4-depleted animals but not in ZIKV/DENV-primed CD4-depleted animals. This study confirms the critical role of CD4⁺ cells in priming an early effective humoral response during sequential flavivirus infections. Our work here suggests that the order of flavivirus exposure affects the outcome of a tertiary infection. Our findings have implications for understanding the complex flavivirus immune responses and for the development of effective flavivirus vaccines.