Seasonal variation in sunlight exposure is differently associated with changes in T regulatory and T-helper 17 cell blood counts in adolescent and adults females: a pilot study.

Higher prevalence of multiple sclerosis at higher latitudes is associated with reduced sunlight during childhood. Alterations in inflammatory Th17 and regulatory T cells (Treg) are associated with autoimmunity. In Hobart, Australia (latitude 42.8° south), thirteen girls (aged 12-13) and sixteen women had blood collected in the evening in daylight in February/March, (post 15 h daylight) and at the same time in the dark in August/September (post 9 h daylight). Height and weight were measured. Participants completed online surveys prior around menstruation, sleep, exercise and time outside. Proportions of Th17 (CD4 + , CXCR3-, CCR4 + , CCR6 + , CD161 +), total Treg (CD4 + , CD25 + , CD127low), naïve Treg (CD45RA + , CD4 + , CD25 + , CD127low) and memory Treg (CD45RA low, CD4 + , CD25 + , CD127low) were enumerated by flow cytometry (Cytek Aurora). Hours spent outside was greater in summer than winter (12.5 v 10.5, p = 0.0003). The %Treg/CD4 + was higher in summer than winter (7.3% vs 7%, p = 0.004) including memory Treg (3.1% vs 2.9%, p = 0.02) for all females (n = 29), and naïve Treg were 13% higher in summer in women (3.2% vs 2.8%, p = 0.0009), whereas %Th17 remained unchanged. In women, a negative correlation between the number of hours spent outside in summer and %Th17 was observed (r = - 0.53, p = 0.035). The %Th17 cells were higher in women than girls (4.9% vs 3.1%, p = 0.001), whereas girls had a higher total %Treg (7.6% vs 6.7%, p = 0.005), consisting of a higher naïve Treg (5.5% vs 3.7%, p = 0.0001) while the women had a higher %memory Treg (3.4% vs 2.6%, p = 0.0001). These light-dependent seasonal differences may influence immune development in adolescents.