Virulence properties and protective efficacy of the capsular polymer of Haemophilus (Actinobacillus) pleuropneumoniae serotype 5

Abstract

The role of the capsule of Haemophilus (Actinobacillus) pleuropneumoniae serotype 5 in bacterial virulence, and the protective efficacy of antibody to serotype 5 capsule was investigated. Encapsulated H. pleuropneumoniae serotype 5 were resistant to killing by complement and antibody to capsule or somatic antigens, whereas a noncapsulated mutant was sensitive to killing by the alternative complement pathway alone. Antiserum to whole H. pleuropneumoniae serotype 5 bacteria or monospecific antiserum to capsule was capable of opsonizing bacteria of the homologous serotype for phagocytosis by swine polymorphonuclear leukocytes but was not opsonic for a heterologous serotype. An immunoglobulin M monoclonal antibody to the serotype 5 capsule was not opsonic for any serotype. Mice were protected against lethal, intranasal challenge with the homologous or heterologous serotype after immunization with live encapsulated or noncapsulated bacteria, but not after immunization with killed bacteria, lipopolysaccharide, or a capsule-protein conjugate vaccine. The protection induced by immunization with live bacteria was transferred to nonimmune, syngeneic mice by serum but not by spleen cells. Nonimmune pigs passively immunized with monospecific swine serum to capsule were protected from lethal infection but not from development of hemorrhagic lung lesions, whereas pigs passively immunized with swine antiserum to live bacteria did not develop severe respiratory lesions. Thus, the capsule of H. pleuropneumoniae serotype 5 was inhibitory to the bactericidal activity of serum and was antiphagocytic. Antibody to the capsule was opsonic but was not fully protective.

Efficacy of a cell extract from Actinobacillus (Haemophilus) pleuropneumoniae serotype 1 against disease in swine

“…Capsular polysaccharides are serotype specific and poorly immunogenic (18). Protection with partially purified outer membrane proteins (OMPs) and lipopolysaccharides was comparable with that of current commercial products (10)(11)(12)33), and partial protection was reported for a capsular polymer (17).…”

“…Serotypes 1, 5, and 7 are the predominant serotypes isolated from A. pleuropneumoniae outbreaks in the United States. One approach to controlling infections has been vaccination with whole-cell bacterins (17,23,35); however, results have been less than optimal. Current vaccines are composed of a combination of chemically inactivated A. pleuropneumoniae adjuvant-treated serotypes (12,29) which may confer immunity to the bacterial cells but not necessarily to secreted products that are involved in pathogenesis.…”

Galactose-1-phosphate uridyltransferase (GalT), an in vivo-induced antigen of Actinobacillus pleuropneumoniae serovar 5b strain L20, provided immunoprotection against serovar 1 strain MS71

“…The murine model for APP respiratory tract infection has been experimentally developed to evaluate the efficacy of vaccines [1]. According to previous studies, this model is a far less laborious and resource intensive than the standard porcine model [2,3]. 15 serovars have been reported over the past several decades, and a new serovar 16 was reported in 2017 [4,5].…”

“…The most promising vaccine candidates are cross protective antigens; typically these are immunogenic markers of pathogens highly conserved across multiple serovars [11]. Virulence factors of APP like Apx toxins, lipopolysaccharide, and capsular polysaccharides have been studied as cross-protective vaccine candidates [10,12,13]. ApxI, ApxII and ApxIII have proven to provide cross protection against different APP serovars [7,10,12].…”

Actinobacillus pleuropneumoniae metalloprotease: cloning and in vivo expression

“…The clinical and pathological effects of both natural and experimental infections are well documented [1,2]. It seems likely that certain virulence factors are involved, such as capsular polysaccharides, lipopolysaccharide (LPS), cytotoxins, outer membrane proteins, fimbriae and enzymes [3][4][5][6].…”