Differences in degree of environmental exposure to antigens in early life have been hypothesized to lead to differences in immune status in individuals from different populations, which may have implications for immune responses in later years.

Venous blood from HIV-negative adolescents and blood from the umbilical cords of babies, born to HIV-negative women, post-delivery was collected and analysed using flow cytometry. T cell phenotype was determined from peripheral blood lymphocytes and cytomegalovirus (CMV) seropositivity was assessed by ELISA in adolescents.

HIV-negative Malawian adolescents were shown to have a lower percentage of naïve T cells (CD45RO^-CD62L^hi CD11a^lo), a higher proportion of memory T cells and a higher percentage of CD28^- memory (CD28^-CD45RO⁺) T cells compared to age-matched UK adolescents. Malawian adolescents also had a lower percentage of central memory (CD45RA^-CCR7⁺) T cells and a higher percentage of stable memory (CD45RA⁺CCR7^-) T cells than UK adolescents. All of the adolescents tested in Malawi were seropositive for CMV (59/59), compared to 21/58 (36%) of UK adolescents. CMV seropositivity in the UK was associated with a reduced percentage of naïve T cells and an increased percentage of CD28^- memory T cells in the periphery. No differences in the proportions of naïve and memory T cell populations were observed in cord blood samples from the two sites.

It is likely that these differences between Malawian and UK adolescents reflect a greater natural exposure to various infections, including CMV, in the African environment and may imply differences in the ability of these populations to induce and maintain immunological memory to vaccines and natural infections.