CCR2 and bacterial infectious disease: In the pathological situation of mouse orchitis caused by viral or bacterial infection or autoantibodies, monocytes (Ly6C+ CCR2+ CX3CR1low, equivalent to human classical monocytes) expressing CCL2 receptors (i.e., CCR2+) in the blood flow into the testes in response to inflammatory stimuli and then differentiate into classical M1 macrophages [19,21] (Figure 1).