O26 Polysaccharides as Key Players in Enteropathogenic E. coli Immune Evasion and Vaccine Development.

Enteropathogenic Escherichia coli (EPEC) produce a capsule of polysaccharides identical to those composing the O-antigen polysaccharide of its LPS (lipopolysaccharide) molecules. In light of this, the impact of O26 polysaccharides on the immune evasion mechanisms of capsulated O26 EPEC compared to non-capsulated enterohemorrhagic Escherichia coli (EHEC) was investigated. Our findings reveal that there was no significant difference between the levels in EPEC and EHEC of rhamnose (2.8:2.5), a molecule considered to be a PAMP (Pathogen Associated Molecular Patterns). However, the levels of glucose (10:1.69), heptose (3.6:0.89) and N-acetylglucosamine (4.5:2.10), were significantly higher in EPEC than EHEC, respectively. It was also observed that the presence of a capsule in EPEC inhibited the deposition of C3b on the bacterial surface and protected the pathogen against lysis by the complement system. In addition, the presence of a capsule also protected EPEC against phagocytosis by macrophages. However, the immune evasion provided by the capsule was overcome in the presence of anti-O26 polysaccharide antibodies, and additionally, these antibodies were able to inhibit O26 EPEC adhesion to human epithelial cells. Finally, the results indicate that O26 polysaccharides can generate an effective humoral immune response, making them promising antigens for the development of a vaccine against capsulated O26 E. coli.

O26 Polysaccharides as key players in enteropathogenic E. coli immune evasion and vaccine development

Enteropathogenic Escherichia coli (EPEC) produce a capsule of polysaccharides identical to those composing the O-antigen polysaccharide of its LPS (lipopolysaccharide) molecules. In light of this, the impact of O26 polysaccharides on the immune evasion mechanisms of capsulated O26 EPEC compared to non-capsulated enterohemorrhagic Escherichia coli (EHEC) was investigated. Our findings reveal that there was no significant difference between the levels in EPEC and EHEC of rhamnose (2.8:2.5), a molecule considered to be a PAMP (Pathogen Associated Molecular Patterns). However, the levels of glucose (10:1.69), heptose (3.6:0.89) and N-acetylglucosamine (4.5:2.10), were significantly higher in EPEC than EHEC, respectively. It was also observed that the presence of a capsule in EPEC inhibited the deposition of C3b on the bacterial surface and protected the pathogen against lysis by the complement system. In addition, the presence of a capsule also protected EPEC against phagocytosis by macrophages. However, the immune evasion provided by the capsule was overcome in the presence of anti-O26 polysaccharide antibodies, and additionally, these antibodies were able to inhibit O26 EPEC adhesion to human epithelial cells. Finally, the results indicate that O26 polysaccharides can generate an effective humoral immune response, making them promising antigens for the development of a vaccine against capsulated O26 E. coli.

Potencial do polissacarídeo O26 para ser utilizado como antígeno em formulações vacinais contra a-EPEC e STEC

According to the World Health Organization, diarrheal diseases are the second leading cause of mortality in the world in children under 5 years of age, with diarrheagenic Escherichia coli being the main etiological agent of bacterial origin. This group of E. coli is also responsible for outbreaks of bloody diarrhea and hemolytic uremic syndrome (HUS) in developed countries. Among the main groups of diarrheagenic E. coli are the strains belonging to serogroup O26 that can be categorized as EPEC (enteropathogenic E. coli) and STEC (shiga toxin producing E. coli). Currently, the best way to combat these pathogens is to develop a vaccine that is effective against both categories belonging to serogroup O26. Faced with this problem, this study aimed to determine the potential of the O26 polysaccharide, present in EPEC and STEC strains, to be used as a target antigen in vaccine formulations against these pathogens. For this, we used two different strains of serotype O26:H11, one from the EPEC category and the other from the STEC category. Being EPEC, capsulated bacteria, we determined the ability of anti-O26 antibodies to recognize capsulated and non-encapsulated bacteria. We also verified the ability of these antibodies to recognize bacteria in the presence of biofilm. Finally, we determined the ability of anti-O26 antibodies to increase EPEC phagocytosis by macrophages and inhibit the adhesion of these pathogens to epithelial cells. The results obtained by the ELISA technique showed that the level of recognition of biofilm-forming EPEC by anti-O26 antibodies was equivalent to the recognition of non-biofilm-producing STEC. The results also showed that the antibodies helped macrophages (J774A.1) in the phagocytosis of EPEC and were able to inhibit the adhesion of these bacteria to epithelial cells. In summary, the results indicate that the O26 polysaccharide is a good antigen candidate to be used in vaccine formulations against all E. coli strains belonging to the O26 serogroup, regardless of their virulence mechanism.

Segundo a Organização Mundial da Saúde, as doenças diarreicas são a segunda maior causa de mortalidade no mundo em crianças menores de 5 anos, sendo Escherichia coli diarreiogênicas, o principal agente etiológico de origem bacteriana. Esse grupo de E. coli também é responsável por surtos de diarreia com sangue e síndrome hemolítica urêmica (SHU) em países desenvolvidos. Dentre os principais grupos de E. coli diarreiogênicas encontram-se as linhagens pertencentes ao sorogrupo O26 que podem ser categorizadas como EPEC (E. coli enteropatogênica) e STEC (E. coli produtoras de toxina shiga). Atualmente, a melhor maneira de se combater esses patógenos é o desenvolvimento de uma vacina que seja eficaz contra as duas categorias pertencentes ao sorogrupo O26. Diante dessa problemática, esse estudo visou determinar o potencial do polissacarídeo O26, presente nas cepas de EPEC e STEC de ser utilizado como antígeno alvo em formulações vacinais contra esses patógenos. Para isso utilizamos duas linhagens diferentes do sorotipo O26:H11, sendo uma da categoria EPEC e outra da categoria STEC. Sendo as EPEC bactérias capsuladas, determinamos a capacidade de anticorpos anti-O26 de reconhecer bactérias capsuladas e não capsuladas. Verificamos também, a capacidade desses anticorpos de reconhecer bactérias na presença de biofilme. Finalmente, determinamos a capacidade dos anticorpos anti-O26 de aumentar a fagocitose de EPEC por macrófagos e inibir a adesão desses patógenos a células epiteliais. Os resultados obtidos pela técnica de ELISA mostraram que o nível de reconhecimento de EPEC formadora de biofilme pelos anticorpos anti-O26 foi equivalente ao reconhecimento de STEC não produtoras de biofilme. Os resultados também mostraram que os anticorpos auxiliaram macrófagos (J774A.1) na fagocitose de EPEC e também foram capazes de inibir a adesão dessas bactérias a células epiteliais. Em resumo, os resultados indicam que o polissacarídeo O26 é um bom candidato a antígeno para ser utilizado em formulações vacinais contra todas as linhagens de E. coli pertencentes ao sorogrupo O26 independentemente do mecanismo de virulência das mesmas.