FN1 and infection: Our previous opsonization studies (38, 54, 89) and others (6, 27) have demonstrated that infection with virulent strain Schu S4 induces less TLR2-dependent immune responses than that of avirulent strain Fn, including activated ERK (54) and that ligation of CR3 with serum pre-opsonized Francisella resulted in dampening TLR2-mediated ERK activation in human cells (54).