Naturally-acquired immunity to Neisseria meningitidis group A.

Group A meningococcal disease is epidemic in Sudan, less common in Uganda, a country bordering the "meningitis belt," and rare in North America. The basis of naturally-acquired group A immunity is unknown but in North America protection has been attributed to a high prevalence of serum anticapsular antibodies elicited by cross-reacting bacteria. We measured group A anticapsular antibody concentrations and bactericidal titers in sera from 236 adults (47 from the Sudan obtained at the height of a group A epidemic, 57 from Uganda, and 132 from North America). Anticapsular antibody concentrations were higher in Sudanese sera than in North American or Ugandan sera (geometric mean of 31.5 versus 5.4 and 5.3 microg/ml, respectively, P < 0.0001). Bactericidal titers of > or =1:4 (presumed to be a protective titer when measured with human complement) were detected in 66% of Sudanese sera as compared with 27 and 23%, respectively, of North American and Ugandan sera (P < 0.0001). Bactericidal activity was inhibited by group A polysaccharide in 58% of the Sudanese bactericidal sera as compared to 17 and 6% of North America and Ugandan bactericidal sera (P < 0.0005). Approximately 50% of non-bactericidal Sudanese sera had high IgA anticapsular antibody concentrations, which were rare in bactericidal Sudanese sera. Thus, serum anticapsular antibodies and bactericidal activity are prevalent in Sudanese exposed to a group A epidemic. Cross-reacting group A anticapsular antibodies are prevalent in North American and Ugandan sera, but bactericidal activity is infrequent and when present is largely directed at non-capsular antigens.