(a,b) RIPK2 bone marrow chimeras were analyzed at day 2 after PR8 infection for contribution to elevated IL-18 production by using IL-18 immunohistochemistry (a) and IL-18 ELISA (b) of whole lung homogenates. WT, wild type; t.p., total protein. In a, blue arrows denote IL-18+ macrophages, and red arrows denote IL-18+ bronchial epithelium. Original magnification, ×200. 