Js. Boschwitz et Jf. Timoney, INHIBITION OF C3 DEPOSITION ON STREPTOCOCCUS-EQUI SUBSP EQUI BY M-PROTEIN - A MECHANISM FOR SURVIVAL IN EQUINE BLOOD, Infection and immunity, 62(8), 1994, pp. 3515-3520
The effect of the M protein of Streptococcus equi subsp. equi on compl
ement deposition, complement degradation, and bacterial survival in eq
uine whole blood was examined in vitro. Preincubation of bacteria with
rabbit M protein-specific immunoglobulin G (IgG) inhibited the surviv
al of the M(+) strain in whole blood by 20-fold (P < 0.01). In additio
n, preincubation of bacteria with M protein-specific F(ab')(2) fragmen
ts inhibited the survival of M(+) cells in whole blood by 3.8 fold (P
< 0.01). In the absence of specific antibody, an M(+) strain (CF32) of
S. equi subsp. equi survived 100-fold better in whole blood than an M
(-) isolate (strain 19) (P < 0.01). Complement inactivation by cobra v
enom factor significantly enhanced the ability of the M(-) and MC stra
ins of S. equi subsp. equi to survive in whole blood, the latter in th
e presence or absence of M protein-specific IgG. The major opsonic for
ms of C3, C3b and iC3b, were present on both M(-) and M(+) cells after
opsonization in nonimmune plasma. However, colloidal gold staining in
dicated that the M(-) strain bound four times as much C3 as the M(+) s
train (P < 0.02) and that preincubation of the M(+) strain with M prot
ein-specific IgG or F(ab')(2) fragments also enhanced the amount of C3
deposited by a factor of 4 (P < 0.02). Therefore, at least part of th
e M protein's ability to enhance bacterial survival in equine whole bl
ood may be related to its ability to interfere with the deposition of
equine complement on the bacterial surface.