To gain more insight into the molecular basis of ICOS function during the clearance of Chlamydia infection, we analysed immune responses during pulmonary infection of Chlamydia muridarum in ICOS-WT, ICOS-KO, and ICOS-YF mice in a Chlamydia-resistant C57BL/6 background. The gene discussed is ICOS; the disease is chlamydia infectious disease.