Two sides of the same coin: Protective versus pathogenic CD4(+) resident memory T cells

The ability of the adaptive immune system to form memory is key to providing protection against secondary infections. Resident memory T cells (TRM) are specialized T cell populations that reside within tissue sites where they await reencounter with their cognate antigen. TRM are distinct from circulating memory cells, including central and effector memory T cells, both functionally and transcriptionally. Since the discovery of TRM, most research has focused on CD8(+) TRM, despite that CD4(+) TRM are also abundant in most tissues. In the past few years, more evidence has emerged that CD4(+) TRM can contribute both protective and pathogenic roles in disease. A complexity inherent to the CD4(+) TRM field is the ability of CD4(+) T cells to polarize into a multitude of distinct subsets and recognize not only viruses and intracellular bacteria but also extracellular bacteria, fungi, and parasites. In this review, we outline the key features of CD4(+) TRM in health and disease, including their contributions to protection against SARS-CoV-2 and potential contributions to immunopathology associated with COVID-19.