IL-12, IFN-gamma, and T cell proliferation to measles in immunized infants

Measles infection in infants is associated with severe complications, and s econdary infections are attributed to generalized immunosuppression, Measle s binding to its monocyte receptor down-regulates IL-12 which is expected t o diminish Th1-like cytokine responses, including IFN-gamma, Whether young infants can be immunized effectively against measles is an important public health issue. We evaluated Ag-specific IL-12, IFN-gamma, and T cell respon ses of infants at 6 (n = 60), 9 (n = 36), or 12 mo (n = 56) of age and 29 v accinated adults, IL-12 and IFN-gamma release by PBMC stimulated with measl es Ag increased significantly after measles immunization in infants. IL-12 and IFN-gamma concentrations were equivalent in younger and older infants, but IL-12 concentrations were significantly lower in infants than in adults (p = 0.04). IL-12 production by monocytes was down-regulated by measles; t he addition of recombinant human IL-12 enhanced IFN-gamma production by PBM C stimulated with measles Ag, but infant T cells released significantly les s IFN-gamma than adult T cells under this condition, Of particular interest , the presence of passive Abs to measles had no effect on the specific T ce ll proliferation or IFN-gamma production after measles stimulation. Cellula r immunity to measles infection and vaccination may be limited in infants c ompared with adults as a result of less effective IFN-gamma and IL-12 produ ction in response to measles Ags. These effects were not exaggerated in you nger infants compared with effects in infants who were immunized at 12 mo. In summary, infant T cells were primed with measles Ag despite the presence of passive Abs, but their adaptive immune responses were limited compared with those of adults.