The mechanism by which acute MHV68 infection can suppress the generation of a humoral response to malaria during co-infection is unclear, but GC B cells from animals with MHV68 co-infection 15 or 7 days prior to P. yoelii XNL infection had increased expression of PD-L1 (S6 Fig), a ligand for PD-1 that negatively regulates Tfh expansion [31], relative to P. yoelii XNL singly-infected animals (Kruskal-Wallis p<0.05; Dunn’s pairwise comparison p>0.05 in both cases). This evidence concerns the gene PDCD1 and infection.