CCL2 and syphilis: Tp0136, another outer membrane protein of T. pallidum, has the ability to bind the fibronectin of host cells, which plays a crucial role in the pathogenesis of syphilis and promotes the migration of HMEC-1 cells by inducing CCL2 expression via the interaction of the fibronectin RGD domain with integrin β1 and the CCL2/CCR2 signaling pathway, and these interactions may contribute to the mechanisms that increase the capacity for self-healing infection of syphilis (13).