The quality of immune responses develops from birth into adulthood and in the context of the host microbial environment. The aim of this work was to study immune maturation during childhood, and how this process can be affected by the common herpesviruses; Epstein-Barr virus (EBV) and cytomegalovirus (CMV).

In paper I we studied monocytes, an important cell type for immunity in the newborn. We showed that the neonatal monocyte subsets exist in similar frequencies as adult subsets, and have a potent capacity for pro-inflammatory cytokine production. In paper II, III and IV we studied the effects of EBV and CMV infections on immune cell function in children. In paper II we found that monocyte-induced NK-cell production of IFN-γ, and plasma IFN-γ levels, were decreased in 2-year old EBV- and/or CMV-seropositive children and mostly so in co-infected children. In paper III we found that in 5-year old children, EBV and CMV co-infection was associated with the highest levels of differentiated NKG2C⁺ NK cells. CMV⁺ children had higher plasma IFN-γ and IL-15 levels and higher NK-cell cytotoxic capacity. In vitro PBMC systems showed elevated frequencies of NKG2C⁺ NK cells in the presence of EBV-infected cells. In paper IV we showed that a child’s age and subsequent capacity for anti-viral cytokine production affects in vitro EBV infection in terms of B-cell proliferation and B-cell acquisition of memory phenotype. PBMC from CMV⁺ children had lower EBV-induced accumulation of switched memory B cells, which was connected to high prevalence of CD57⁺CD8⁺ T cells and IFN-γ production.

Taken together, this thesis work shows that monocyte subsets at birth can give potent functional responses and that latency with EBV and CMV has a significant effect on the differentiation process and functional capacity of anti-viral effector cells during childhood. This in turn could affect responses to related or unrelated infections or even to non-invasive antigens such as allergens.