Table 1.

Changes to circulating T cell subsets associated with UVR exposure. T cells express CD3, the T cell co-receptor. They are then subdivided according to their phenotype that frequently reflects their function. Classification has often been aided by detection of intracellular cytokine expression after a short stimulation in vitro. CXCR5 allows cell migration into germinal centres.

Cell subset	Phenotype	% of CD3+ cells*	% CD4+ cells*	Changes after UVR exposure
T helper (Th)	CD3+CD4+	60-70		No change (8)
Th1	CD3+CD4+CXCR3+ (T-bet+) making IFNγ on in vitro stimulation		variable	No change (20) Reduced (21)
Th2	CD3+CD4+CXCR3-CCR6-, making IL-4 on in vitro stimulation		variable	No change (20) Reduced (21)
Th17	CD3+CD4+CXCR3-CCR6+ (ROR-γt+) making IL-17 on in vitro stimulation		variable	Increased (20) No change (8) Decreased (21, 22)
Skin-homing Th	CD3+CD4+CLA+CCR4+ (ref)		variable	Decreased (21)
Treg	CD3+CD4+CD25+Foxp3+(sometimes CD27+/CD127lo)		5	No change as % of T cells (8, 20, 23, 24) Increased as % of T cells (16, 25, 26) Increased activated Treg implied by activation-associated phenotype (23) Increased Treg function (25, 26)
T follicular helper	CD3+CD4+CXCR5+		15	No change (8)
T follicular regulatory	CD3+CD4+CXCR5+Foxp3+		1.5	No change (8)
T cytotoxic	CD3+CD8+	20-30		No change (8, 27, 28)

*Percentages in healthy individuals (8).