Newborn BCG Vaccination Induces Robust Infant IFN-Expressing NK Cell Responses to Mycobacteria

Infants are highly susceptible to disease caused by infection with intracellular pathogens, such as Mycobacterium tuberculosis. The Bacille Calmette-Guerin (BCG) vaccine is usually administered at birth to protect against severe forms of tuberculosis in children. BCG also has non-specific protective effects against heterologous infections, thought to be mediated by enhanced innate cell functions. We aimed to determine whether newborn BCG vaccination modulates myeloid and NK cell responses to mycobacteria. We enrolled 130 South African infants in whom BCG vaccination was either administered at birth or delayed to 6 or 10 weeks of age. CD4 T, myeloid and NK cell responses to whole cell mycobacteria were measured by flow cytometry. Levels of cytokines secreted upon whole blood stimulation with BCG were measured by multiplex bead array. Newborn BCG vaccination was associated with significantly higher frequencies of BCG-reactive, cytokine-expressing CD4+ T cells and IFNγ-expressing NK cells, compared with unvaccinated infants at 5-weeks and 9-weeks of age. However, cytokine-expressing CD33+ myeloid cells were not different between vaccinated and unvaccinated infants. Induction of BCG-reactive IFNγ-expressing NK cells was not associated with enhanced NK cell maturation, differentiation or cytokine receptor expression. Although levels of secreted cytokines were not different in vaccinated and unvaccinated infants, BCG-reactive NK cell responses correlated directly with levels of IL-2 and IFNγ, and the innate pro-inflammatory cytokines IL-6, IL-1β and TNF in BCG vaccinated infants only. Our data show that BCG-reactive IFNγ-expressing NK cells are strongly induced by BCG vaccination in infants and are likely amplified through by-stander cytokine-mediated activation. Further studies in infants are needed to improve our understanding of the possible role BCG-induced NK cell responses may play in mediating protection against tuberculosis and other infectious diseases. Funding Statement: NIH (R01AI087915), European and Developing Countries Clinical Trials Partnership (EDCTP, TA.2011.40200.010), South African National Research Foundation (MSc and PhD fellowships to MM). Declaration of Interests: No competing interests. Ethics Approval Statement: Parents or legal guardians provided written, informed consent before enrolment in the study. The Human Research Ethics Committee at the University of Cape Town approved protocols for infants vaccinated at birth (REF: 126/2006) and infants who had delayed BCG vaccination (REF: 177/2011). Good Clinical Practice procedures and the World Medical Association Declaration of Helsinki guidelines were adhered to.