In summary, in all nine combinations of the three C.tr. serovars used in first and second i.n. infections, all acquired parameters of disease (i.e., weight loss, clinical score, and bacterial load in the lung and levels of IFN-γ, TNF-α, as well as MPO) were similarly diminished after secondary infection, indicating strong cross-serovar protection between C.tr. D, E, and L2. Here, IFNG is linked to infection.