A Subset of CD4 + Thymocytes Selected by MHC Class I Molecules

To complete their maturation, most immature thymocytes depend on the simultaneous engagement of their antigen receptor [αβ T cell receptor (TCR)] and their CD4 or CD8 coreceptors with major histocompatibility complex class II or I ligands, respectively. However, a normal subset of mature αβ TCR+ thymocytes did not follow these rules. These thymocytes expressed NK1.1 and a restricted set of αβ TCRs that are intrinsically class I-reactive because their positive selection was class I-dependent but CD8-independent. These cells were CD4 + and CD4 - 8 - but never CD8 + , because the presence of CD8 caused negative selection. Thus, neither CD4 nor CD8 contributes signals that direct their maturation into the CD4 + and CD4 - 8 - lineages.