Antibody and cytokine-associated immune responses to S. equi antigens entrapped in PLA nanospheres

Strangles is an infectious disease caused by Streptococcus equi subspecies equi that affects the upper respiratory tract of the Equidae. The control of this disease seems to be dependent on its earlier detection and prevention, but prolonged animal protection without development of strong and severe side effects has not yet been achieved. Convalescent horses exhibit a protective immune response, mainly against SeM (58 kDa), an antiphagocytic and opsonogenic S. equi M-like protein, known as the major protective antigen against strangles. Purified recombinant SeM and S. equi protein extract-entrapped poly(lactic acid) (PLA) nanospheres were developed and their adjuvant potential was studied via the intramuscular route. The effect including molecules with adjuvant properties such as spermine, oleic acid, alginate and glycol-chitosan was also evaluated. Spherical nanometric particles <500 nm containing the protein antigen were prepared by the solvent evaporation method and protein structure was not affected throughout preparation. The humoral immune response induced by nanospheres was markedly higher than that elicited by soluble antigens, isolated or co-admixed with CpG. The IgG and IgG subtypes, along with cytokine titres, indicated that nanospheres composed by glycolchitosan developed a more balanced Th1/Th2 response for both purified SeM and S. equi enzymatic extract proteins, although those induced by the pure antigen-entrapped particles were higher than the S. equi tested vaccines composed by total antigens entrapped in polymeric nanospheres.