KLRG1 and viral infectious disease: B and T lymphocytes can develop into a state of senescence, which often coexist with downregulated telomerase, shortened telomeres, increased DNA damage response, and increased mitogen-activated protein kinase activity.36, -, 39 Viral infections, such as cytomegalovirus, especially in older individuals, can also induce the expression of senescent markers such as CD57 and KLRG1 on T lymphocytes, so does repetitive antigenic stimulation.