Tuberculosis causes 2 million deaths per year, yet in most cases the immune response successfully contains the infection and prevents disease outbreak. Induced lymphoid structures associated with pulmonary granuloma are observed during tuberculosis in both humans and mice and could orchestrate host defense. To investigate whether granuloma perform lymphoid functions, mice lacking secondary lymphoid organs (SLO) were infected with Mycobacterium tuberculosis (MTB). As in WT mice, granuloma developed, exponential growth of MTB was controlled, and antigen-specific T-cell responses including memory T cells were generated in the absence of SLO. Moreover, adoptively transferred T cells were primed locally in lungs in a granuloma-dependent manner. T-cell activation was delayed in the absence of SLO, but resulted in a normal development program including protective subsets and functional recall responses that protected mice against secondary MTB infection. Our data demonstrate that protective immune responses can be generated independently of SLO during MTB infection and implicate local pulmonary T-cell priming as a mechanism contributing to host defense.Key words: Granuloma . Lymphoid neogenesis . T-cell development . Tuberculosis Supporting Information available online
IntroductionMycobacterium tuberculosis (MTB), the causative agent of tuberculosis (TB), currently persists in one-third of the world's population [1]. In most cases, the immune response generated upon exposure contains, but does not eradicate, the bacilli and an asymptomatic persistent infection develops, termed latent TB. Approximately one in ten latently infected individuals undergo reactivation to active TB disease during their lifetime. However, Ã These authors contributed equally to this work. ÃÃ These authors share equal senior authorship. [2,3]. Granuloma are considered central for control of MTB as loss of granuloma structure is associated with poor disease outcome in humans and animal models [3,4]. Yet, the precise mechanisms by which granuloma mediate control of MTB are poorly understood. Induction of an adaptive immune response is essential for host protection against MTB. Antigen-specific T-cell responses are generated within secondary lymphoid organs (SLO), such as lymph nodes (NO) and spleen. SLO maximize efficiency of encounters between APC and naïve T lymphocytes [5]. Therefore, SLO are critical for the generation of adaptive immunity. Ectopic accumulations of lymphoid cells, known as tertiary lymphoid organs, sometimes arise during chronic inflammation or infection. Tertiary lymphoid organs resemble SLO in structure and cell composition and evidence exists that some are capable of generating adaptive immunity [5,6]. However, whether immune responses generated outside of SLO are truly effective and thus beneficial in defense against pathogens is unclear.Recent reports have described lymphoid features associated with murine and human granuloma, such as B-cell follicles and the peripheral node addressin, which is expressed on high endothel...