Fig. 4 |. Immunosurveillance by CD4⁺ T cells.

Memory CD4⁺ T cell immunosurveillance strategy after a respiratory viral infection that elicits a type I response. Although we focus on memory T_H1 cells, both memory T_H2 and memory T_H17 cells have been well documented to take up permanent residence in NLTs. In contrast to T_H1 T_CM cells, which express CD62L, many memory T_H1 cells cannot access lymph nodes (LN) through high endothelial venules. These memory T_H1 cells in lymph nodes might represent T_H1 cells that recirculate constitutively through NLTs and enter lymph nodes through afferent lymphatics (‘backdoor entry’) from upstream NLTs, or SLO-resident memory T_H1 cells. Memory T_H1 cells in the blood comprise blood restricted cells, NLT recirculators and T_CM cells. FR4^hi memory T_FH cells populate SLOs and may reside at the outer boundaries of B cell follicles. T_FH-like CD4⁺ T_RM cells have been identified in lungs of influenza-infected mice. In contrast to resident T_H1 cells, resident T_FH cells seem locally restricted to TLSs.