The higher prevalence of multiple sclerosis at higher latitudes is associated with reduced sunlight during childhood, and female sex. Alterations in inflammatory Th17 and regulatory T-cells are associated with immune auto-reactivity, with regulatory T-cells being important in suppressing reaction against self tissue. In Hobart, Australia (latitude 42.80 south), thirteen girls (aged 12-13) and fifteen mothers (aged 34-55) had blood collected in the evening in daylight in February/March, (prior daylight 13-15 hours) and at the same time in the dark in August/September (9-11 hours 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), relative to CD4+ T-cells, were enumerated by flow cytometry (Cytek Aurora).
Hours spent outside were greater in summer than winter (12.5 v 10.5, p=0.0003), the %Treg was higher in summer than winter (7.3% vs 7%, p=0.002), including memory Treg (3.1% vs 2.9%, p=0.01), and naïve Treg (4.2% vs 4.0%, p=0.03), 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 was 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 greater % naïve Treg (5.5% vs 3.7%, p=0.0001) whereas 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 and future auto-reactivity.