CCL2 and infection: Finally, when U937 cells were treated 24 h post infection there was an increase in MCP-1 production at 48 h [1.23-fold reduction of MCP-1 following NaHS treatments (p = ns) and 2.35-fold reduction following GYY4137 treatments (p = 0.001)], suggesting that at this time point the protective effect of H2S is reduced.