Immune responses to the lipopolysaccharides and capsular polysaccharides of Haemophilus pleuropneumoniae in convalescent and immunized pigs

Abstract

The immunologic responses to a smooth-type lipopolysaccharide (LPS) (HpS-LPS), a rough-type LPS (HpR-LPS), and a capsular-enriched polysaccharide preparation (HpC-PS) purified from Haemophilus pleuropneumoniae were determined in pigs immunized with a commercial H. pleuropneumoniae cellular vaccine, in pigs experimentally infected with H. pleuropneumoniae, in control pigs, and in immunized rabbits. The ability of the preparations to induce lymphocyte blastogenesis and B-cell activation was determined in the pigs and compared with the responses induced by the LPS of Escherichia coli O111:B4 and the LPS of Salmonella minnesota Re595. All the LPS preparations acted to induce proliferation of peripheral blood lymphocytes (PBL) from all pigs. The blastogenic response of PBL from H. pleuropneumoniae-infected pigs to HpS-LPS and HpR-LPS was significantly (P less than 0.05) greater than that of PBL from immunized and control pigs. HpC-PS did not induce a blastogenic response in the PBL of control pigs but did in PBL from H. pleuropneumoniae-infected pigs and to a greater degree in immunized pigs. An increase in the response of PBL to the S. minnesota LPS occurred only in the H. pleuropneumoniae-infected pigs. Significantly more (P less than 0.05) immunoglobulin-secreting cells (ISC) were induced in a reverse hemolytic plaque assay by stimulation with HpS-LPS and HpC-PS of PBL isolated from pigs infected with H. pleuropneumoniae than of PBL from immunized pigs. Increasing the number of T cells increased the number of ISC induced by HpS-LPS in control and immunized pigs, but not in convalescent pigs. The presence of macrophages reduced activation of ISC by HpS-LPS in control pigs and to a lesser degree in immunized pigs, whereas in H. pleuropneumoniae-infected pigs macrophages enhanced the induction of ISC by HpS-LPS. In immunized pigs, macrophages acted to inhibit the ability of HpC-PS to induce ISC. Serologic studies indicate that HpC-PS contains strain- and serotype-specific antigens; that HpS-LPS has both serotype-specific and cross-reacting species-specific antigens; and that HpR-LPS does not contain detectable serotype-specific antigens but does have both non species- and species-specific antigens. These studies show that the serotype-specific protection provided by immunization of pigs with an H. pleuropneumoniae cellular vaccine is principally the result of immunity to capsular antigens and that a weak cellular immune response occurs as compared with that induced by infection with H. pleuropneumoniae.(ABSTRACT TRUNCATED AT 400 WORDS)