Chemotactic response of equine polymorphonuclear leucocytes to Streptococcus equi

Streptococcus equi infection in horses is characterised by intense infiltration of lymph nodes by polymorphonuclear leucocytes (PMNs) suggesting a potent chemotactic response to the organism or its products. Equine PMNs were separated using Ficoll-Hypaque medium and used in an assay of chemotaxis under agarose to study the components of S equi involved in this response. Results showed that complement-derived chemotactic factors generated by activation of the alternative complement pathway were important in chemotactic responses to S equi. Both whole bacteria and peptidoglycan preparations were potent complement activators, whereas purified M protein was less active. In contrast, S equi culture supernatant protein did not activate complement; instead it directly inhibited migration of PMNs. Moreover, PMNs, when incubated with culture supernatant of a non-haemolytic strain, showed signs of cellular degeneration suggesting the presence of a cytotoxin distinct from haemolysin.