Early appearance of "natural" mucosal IgA responses and germinal centers in suckling mice developing in the absence of maternal antibodies.

We have examined the role of passively transferred maternal Abs in the ontogeny of "natural" mucosal IgA responses before weaning of suckling mice by comparing the immune status of gut-associated lymphoid tissue (GALT) (Peyer's patches, mesenteric lymph nodes, and lamina propria) in 7- to 25-day-old F1 severe combined immunodeficient (scid)/+ mice generated through reciprocal crosses of C.B17 scid/scid and normal congenic (+/+) adult mice. We have also examined the ability of prenatal vs postnatal transfer of maternal immunity to forestall the development of natural neonatal mucosal IgA responses by swapping litters of F1 scid/+ pups at birth between +/+ and scid/scid mothers. Our results demonstrate that F1 scid/+ pups born to or nursed by scid/scid mothers undergo an accelerated development of natural IgA responses that include germinal center reactions in both Peyer's patches and mesenteric lymph nodes. These early IgA responses are evident as: 1) increased frequencies of IgA-producing GALT organ cultures; 2) increased mean IgA output by GALT organ cultures; 3) increased frequencies (> 1 log) of IgA-secreting cells from GALT detected by ELISPOT at 16 days of age; and 4) germinal center development by 17 days of age detected by in vivo bromodeoxyuridine incorporation. Finally, FACS analyses of enteric bacteria isolated from F1 scid/+ pups and stained for the presence of surface-bound mouse IgA demonstrate that the bacterial flora is a major target of both maternal secretory IgA and of the earliest IgA Abs produced in the neonatal GALT of pups deprived of maternal immunity.