Altered naive CD4 and CD8 T cell homeostasis in patients with relapsing‐remitting multiple sclerosis: thymic versus peripheral (non‐thymic) mechanisms

Summary We have reported previously that naive T cells from relapsing‐remitting multiple sclerosis (RRMS) patients have T cell receptor (TCR) repertoire shifts, but the basis of these TCR repertoire shifts was uncertain. Here, we questioned whether RRMS patients have altered naive CD4 and CD8 T cell homeostasis by studying homeostatic proliferation and thymic production in RRMS patients and healthy controls. We measured thymic production by quantifying signal joint T cell receptor excision circles (sjTRECs). Both naive T subsets from controls showed an age‐associated decrease in sjTRECs, i.e. evidence of progressive thymic involution, but we detected no age‐associated decrease in sjTRECs in RRMS patients. Instead, naive CD8 T cells from patients had lower sjTRECs (P = 0·012) and higher Ki‐67 proliferation levels (P = 0·04) than controls. Naive CD4 T cell sjTRECs did not differ between patients and controls. However, in RRMS these sjTRECs correlated strongly with CD31, a marker expressed by newly generated CD4 T cells but not by naive CD4 T cells that have undergone homeostatic proliferation. HLA‐DR2 positivity correlated negatively with naive CD4 T cell CD31 expression in RRMS (P = 0·002). We conclude in RRMS that naive T subsets have homeostatic abnormalities due probably to peripheral (non‐thymic) mechanisms. These abnormalities could have relevance for MS pathogenesis, as naive T cell changes may precede MS onset.