ISOTYPE-SPECIFIC ANTIBODY-SECRETING CELLS TO TRANSMISSIBLE GASTROENTERITIS VIRUS AND PORCINE RESPIRATORY CORONAVIRUS IN GUT-ASSOCIATED AND BRONCHUS-ASSOCIATED LYMPHOID-TISSUES OF SUCKLING PIGS

Antibody-secreting cells (ASC) were enumerated in gut-and bronchus-associated lymphoid tissues of pigs exposed to three antigenically related coronaviruses: virulent transmissible gastroenteritis virus (TGEV), attenuated TGEV, and porcine respiratory coronavirus (PRCV). Exposure of 11-day-old pigs to virulent TGEV resulted in severe gastroenteritis and virus shedding mainly in feces but also to a limited extent in nasal secretions. PRCV and attenuated TGEV exposure produced no clinical signs and only one pig given a high dose of attenuated TGEV shed virus in feces, but virus was shed from the nasal passages. Nasal virus titers were highest after PRCV inoculation of pigs. Mononuclear cells were isolated from spleens, mesenteric, and bronchial lymph nodes of pigs and assayed forvirus-specific IgG and IgA antibody secretion by an enzyme-linked immunospotassay. Virus-specific ASC peaked at postinoculation days 12 to 24 and IgG-ASC outnumbered IgA-ASC in all tissues tested. The greatest numbers of ASC were in mesenteric lymph nodes of virulent TGEV-exposed pigs and in BLN of PRCV-exposed pigs. Attenuated TGEV induced intermediate ASC responses in the gut and respiratory tract. Secondary in vitro ASC responses to inactivated TGEV or PRCV paralleled the primary responses except in BLN where the numbers of memory ASC were high for both TGEV- and PRCV-exposed pigs. We conclude that: 1) a single exposure of pigs to PRCV either oral-nasally or by aerosol leads to potent systemic and bronchus-associated, but not gut-associated, ASC responses; 2) a high dose of attenuated TGEV (4 x 10(8) plaque-forming units) is more effective than PRCV (6 x 10(5) or 2 x 10(8) plaque-forming units) or a lower dose of attenuated TGEV (7 x 10(6) plaque-forming units) in eliciting gutassociated ASC; 3) although virulent and a high dose of attenuated TGEV induce high numbers of ASC in the tissues tested, virulent TGEV induces the most ASC in, the gut and IgA-ASC in all lymphoid tissues; and 4) virus replication in the gut or respiratory tract is a major factor affecting the magnitude of an ASC response at that site and may be necessary for the recruitment of IgG- and IgA-ASC and memory cells in large numbers from other mucosai inductive sites. This unique model of mucosal immunity using antigenically related viruses with distinct tissue tropisms may help to clarify interactions of the various components of the common mucosal immune system.