Interferon gamma production in HIV-1 exposed uninfected infants
Anthony, Fiona Sharon
Abstract The immaturity of the neonatal immune system places infants at an increased risk of infections and also affects the effective induction of protective immune responses by vaccines. In utero sensitisation to infectious pathogens results in immune activation and can establish immunological memory and may influence the immune response to unrelated antigens. In this study, we investigated in early life (birth to 6-10 weeks) the development of interferon-gamma (IFN-g) responses in uninfected infants born to HIV-1 infected mothers (exposed uninfected (EU) infants). Whole blood cell cultures stimulated with phytohaemagglutinin (PHA) or Mycobacterium bovis bacillus Calmette-Guérin (BCG) showed that EU infants have a greater ability to produce IFN-g in response to PHA and BCG at birth compared to control infants (born to HIV-1 uninfected mothers). However, by six weeks of age control infants produced significantly more IFN-g in response to PHA only. These results suggest that responses amongst EU infants establish and mature earlier than in control infants. In fact, over time a greater number of EU infants have a reduced ability or an inability to respond to stimuli such as PHA or BCG compared to control infants (when comparing responses at six weeks of age to responses of the matched birth samples). Full blood counts (FBC) counts were carried out using an automated AcT 5 diff haematology analyser which measures proportions and absolute counts of the five groups of white blood cells, namely, lymphocytes, monocytes, neutrophils, eosinophils and basophils. Importantly, there were no significant differences in the absolute lymphocyte counts of control infants and EU infants either at birth or at six weeks which could account for the IFN-g production differences noted between these infant groups, although the EU infants did exhibit a trend of higher absolute lymphocyte counts at six weeks than control infants. Age-dependent maturational changes in cell counts were observed in both control and EU infant groups, with neutrophils predominating at birth and lymphocytes predominating by six weeks, indicative of immune development with age in both infant groups. Short-course antiretroviral exposure increased basophil counts of infants at birth but did not affect counts by six weeks. Flow Cytometry studies using an Intracellular Cytokine (ICC) assay were conducted to establish which mononuclear cell types are predominantly responsible for producing IFN-g in infants. ICC assays done on whole blood revealed that natural killer cells (NK) were predominantly responsible for the IFN-g produced by 10 week old EU infants, and also at birth (control and EU infants). At birth whole blood cultures of 48% of EU infants already showed BCG-induced IFN-g responses (prior to BCG vaccination); this could be explained by a non-specific response (NK cells) to the antigen but could also involve T cell responses (CD4+ and/or CD8+ T cells) as supported by ICC data obtained from control and EU newborn infants. The infant immune system was clearly unique from that of their mothers. In particular, mothers’ demonstrated significant changes in blood counts from labour to six weeks postpartum indicative that immuno-modulation plays an essential role in a successful pregnancy. The single dose of nevirapine (sdNVP) taken by the mother at the onset of labour did not influence maternal blood counts significantly and maternal CD4+ and CD8+ T cells, and NK cells produced significantly more IFN-g than the CD14+ monocytes and CD19+ B cells, with CD8+ T cells producing the most. Our results have shown that EU infants are distinct from control infants with respect to immunological responses as measured by IFN-g production, and that maturational differences do exist between control and HIV-1 exposed infants. While the clinical importance of these results remains undetermined it is important to establish whether such immunological changes may result in altered susceptibility to infectious diseases in this already vulnerable population, and how this may impact on the induction of protective immune responses by vaccines. It remains important to identify neonatal immune system deficiencies and understand the consequences of exposure to maternal HIV-1 (in the absence of acquiring HIV-1 infection), the understanding of which could contribute to the development of more effective vaccines to HIV-1 and other infectious diseases.