To decipher which Yop was responsible for the early inhibition of KC, MIP-2 and G-CSF up-regulation, we used two different Yop-null derivatives of the fully virulent Y. pestis strain Kim53, and we performed a series of in vitro infection experiments using alveolar-derived macrophages (MH-S) and lung epithelial (TC-1) cell lines. The gene discussed is CALCA; the disease is infection.