HLA-DR is a molecule typically expressed by antigen-presenting cells and is associated with antigen presentation. Interestingly HLA-DR is also expressed on T cells and has been associated with T cell activation. However, why and how activated T cells express HLA-DR is not fully understood. A recent study by Tippalagama et al. aimed to characterize the phenotype of effector CD4 T cells in active Tuberculosis.
Transcriptomic profiling of non-naïve CD4 T cells from M.tb uninfected, M.tb infected and individuals with active TB identified significantly higher expression levels of molecules associate with antigen-specific T cell activation and inflammation, and specifically identified higher expression of HLA-DRB1 in TB patients compared with both M.tb uninfected and infected individuals. As observed by others, researchers demonstrated higher frequencies of HLA-DR+ circulating M.tb-specific CD4 T cells in tuberculosis patients compared with both M.tb uninfected and infected individuals. Further, HLA-DR+ M.tb-specific CD4 T cells predominantly expressed phenotypic markers associated with an effector memory phenotype (CD45RA-CCR7-) and increased expression of cytotoxic molecules and proinflammatory cytokines (TNF). Lastly, they demonstrated that proliferating T cells express high levels of HLA-DR, suggesting that HLA-DR could be used as a proxy to identify recently divided CD4 T cells upon M. tuberculosis Ag exposure.
Researchers concluded HLA-DR expression is a marker of such effector cells and that HLA-DR+ CD4 T cells were increased in individuals with ATB compared with M.tb uninfected and infected individuals. Further, they suggest that HLA-DR might be a useful marker for identifying effector T cells and monitoring immune responses not only in the context of TB but also in many other infection and vaccination models.