Classical and γδ T cells are each independently sufficient to establish protection against a classical strain of Klebsiella pneumoniae.

Infections with classical strains of the Gram-negative bacterium Klebsiella pneumoniae pose a significant clinical challenge due to rising antibiotic resistance. We previously established a lung inoculation plus challenge model using live, classical K. pneumoniae in order to study host protection. Here, we employ this model to dissect adaptive immune responses to this critical pathogen. First, we performed convalescent serum transfers from inoculated mice to naïve recipients and found that classical K. pneumoniae infection outcomes, unlike hypervirulent K. pneumoniae infection outcomes, were not improved. This suggests that circulating antibody responses alone are not sufficient to mediate protection against this classical strain. Hence, we evaluated the role of T cells in protection against classical K. pneumoniae reinfection and demonstrated that mice lacking T cells are unable to establish a protective response. However, mice individually deficient in either of the major T cell subsets, γδ or αß (classical T cells), effectively mount a protective response, indicating either subset alone is sufficient to mediate protection. Sequestration of T cells in secondary lymphoid organs during the challenge infection did not ablate protection, indicating the circulating T cell pool is not required for the protective phenotype. Finally, we demonstrate that depletion of T cells during initial infection eliminates protection against challenge. Collectively, these experiments demonstrate the imperative contribution of T cells to protective immunity against classical K. pneumoniae and will guide further inquiries into host effector responses required to control this infection.

Murine Respiratory Tract Infection with Classical Klebsiella pneumoniae Induces Bronchus-Associated Lymphoid Tissue.

Klebsiella pneumoniae is a Gram-negative, opportunistic pathogen that commonly causes nosocomial pneumonia, urinary tract infection, and septicemia. Our recent work utilizing a murine model of respiratory tract infection with classical K. pneumoniae demonstrated leukocyte aggregates in the lungs of mice at 28 days postinfection. Here, we sought to characterize the composition and development of these structures. Histopathological analyses of murine lungs revealed immune cell clusters surrounding the pulmonary vasculature and airways by 14 days postinfection, resembling inducible bronchus-associated lymphoid tissue (iBALT). Further investigation of these structures demonstrated central B cell aggregates with concomitant dispersed T cells. At day 28 postinfection, these lymphoid clusters expressed germinal center markers and CXCL12, qualifying these structures as iBALT with nonclassical B cell follicles. Investigations in mutant mice revealed that those lacking B and/or T cells were not able to form fully defined iBALT structures, although some rudimentary B cell clusters were identified in mice lacking T cells. The longevity of K. pneumoniae-induced BALT was assessed for up to 120 days postinfection. Lymphoid aggregates significantly decreased in size and quantity by 90 days after K. pneumoniae infection; however, aggregates persisted in mice that were restimulated with K. pneumoniae every 30 days. Finally, infections of mice with an array of classical K. pneumoniae clinical isolates demonstrated that the development of these structures is a common feature of K. pneumoniae lung infection. Together, these data confirm that murine lungs infected with K. pneumoniae develop iBALT, which may play a role in pulmonary immunity to this troublesome pathogen.