Age-dependent changes in sensitivity to antigen: its relationship with the antigen processing system.

The relationship between the antigen processing or accessory cell system and the maturation, with age, of antibody-producing capability was investigated in the mouse. This was done by analyzing the antibody responses given by immunocompetent cells from neonatal and from adult male mice in an identical antigen-processing system environment. Specifically, 4 x 10(7) normal spleen cells from either 12-day-old or adult mice were challenged with varying numbers of SRC in adult irradiated syngeneic recipients. The subsequent IgM and IgG2a PFC responses for both age groups were analyzed in terms of antigen dose-antibody response curves. Analyses of these curves indicate: 1) the dose of antigen required to elicit the optimal antibody response is essentially identical for both age groups and (2) the bandwidths obtained using neonatal donors are significantly narrower than those obtained with adult donors. Whereas, intact neonatal and adult mice exhibit differences in antigen optimal doses, these differences are eliminated when the immunocompetent cells are stimulated in the presence of identical antigen-processing systems. It is concluded that maturation of the antigen-processing system results in an increased sensitivity to antigen. Examination of the bandwidths of the dose-response curves revealed that immunocompetent cells from the young mice, either in situ or in adult irradiated recipients exhibited narrower bandwidths than did their adult counter parts. Thus, the increase in bandwidth observed with age is attributed to changes in the population of immunocompetent cells--perhaps a reflection of increased diversity, and is not due to the antigen-processing system. Quantitation of the antigen-processing function using accessory cell-depleted and partially restored mice indicated that when IgG responses were compared a higher frequency of accessory cells was demonstrated in adultspleens as opposed to neonatal spleens.