Somatic Hypermutation in T-Independent and T-Dependent Immune Responses toHaemophilus influenzaeType b Polysaccharide

Secondary immune responses to T-independent antigens are characterized by little or no affinity maturation, a phenomenon attributed to limited somatic hypermutation. In the human immune response toHaemophilus influenzaetype b capsular polysaccharide, however, there are numerous differences between rearranged heavy chain variable region gene segments and candidate germline genes, irrespective of antigen presentation in a T-independent or T-dependent form. To determine the characteristics of somatic hypermutation in this response, we analyzed rearranged heavy chain variable region segments and associated 3′ untranslated JH4–JH5 introns from monoclonal anti-Hib PS antibodies. Mutation of untranslated introns and heavy chain variable segments in both T-independent and T-dependent responses resembles that described in murine and unselected human immune responses. Although mutation is frequent in both T-independent and T-dependent anti-Hib PS responses, there is little evidence of antigen-driven selection, suggesting that ongoing pressure to conserve the variable segment germline configuration limits affinity maturation in this immune response.