CD4 and infection: In a different study, mixed bone marrow chimeras revealed dramatically fewer Icos−/− LLOp:I-Ab-binding T cells (listeriolysin O-specific CD4+ T cells) adopted a central memory phenotype at day 20 post-Lm-2W (L. monocytogenes producing the 2W peptide and a fragment of listeriolysin O) infection relative to wild-type LLOp:I-Ab-binding T cells (109).