INTRODUCTION 
Gram-positive bacterium Streptococcus equi subspeices equi (S. equi) causes equine strangles, a highly contagious purulent lymphadenitis and one of the most common infectious diseases in horses [1,2].
The infection initially causes nasal discharge and fever and, then, leads to abscess formation in local lymph nodes, causing enormous pain and respiratory difficulty.
There is massive infiltration of polymorphonuclear leukocytes (PMNs) to the infection site.
However, S. equi effectively evades the horse innate immunity by being resistant to phagocytosis by PMNs.
Horses recovered from strangles acquire immunity against S. equi reinfection [3].
The immunity is primarily mediated by protective antibodies [4], which opsonize and thus enhance phagocytosis of S. equi by horse PMNs.
To survive in hosts, bacterial pathogens have evolved multiple mechanisms to evade host defense.
For examples, both S. equi and group A Streptococcus (GAS) produce the hyaluronic acid capsule and surface protein M protein to contribute to resistance to phagocytosis by PMNs.
We found that GAS produces a secreted Mac protein (also known as IdeE), which inhibits opsonophagocytosis of GAS by human PMNs [5].
This protein can cleave the heavy chain of human immunoglobulin G (IgG) using Cys94, His262 and Asp284 as its catalytic triad [6-8].
There are two kinds of Mac produced by GAS isolates [7], which use different mechanisms to block the interaction between IgG and Fc receptor on the surface of PMNs.
The type-1 Mac, such as M1 Mac produced by serotype M1 GAS strains, has high enzymatic activity and low affinity to Fc receptor on the surface of PMNs, while the type-2 Mac can bind to the Fc receptor and has lower enzymatic activity [9].
S. equi has a homologue of GAS M1 Mac (designated SeMac).
In this study, SeMac was prepared and characterized.
The results indicate that SeMac is a cysteine endopeptidase but does not inhibit opsonophagocytosis of S. equi by horse PMNs, suggesting that SeMac has function other than evading horse acquired immunity against S. equi infection.
