Development of a secondary immune response to Mycobacterium tuberculosis is independent of Toll-like receptor 2

Published work indicates that the contribution of Toll-like receptor 2 (TLR2) to host resistance during acute Mycobacterium tuberculosis infection is marginal. However, in these studies, TLR2 participation in the memory immune response to M. tuberculosis was not determined. The substantial in vitro evidence that M. tuberculosis strongly triggers TLR2 on dendritic cells and macrophages to bring about either activation or inhibition of antigen-presenting cell (APC) functions, along with accumulating evidence that memory T cell development can be calibrated by TLR signals, led us to question the role of TLR2 in host resistance to secondary challenge with M. tuberculosis. To address this question, a memory immunity model was employed, and the response of TLR2-deficient (TLR2 knockout [TLR2KO]) mice following a secondary exposure to M. tuberculosis was compared to that of wild-type (WT) mice based on assessment of the bacterial burden, recall response, phenotype of recruited T cells, and granulomatous response. We found that upon rechallenge with M. tuberculosis, both WT and TLR2KO immune mice displayed similarly enhanced resistance to infection in comparison to their naïve counterparts. The frequencies of M. tuberculosis-specific gamma interferon (IFN-γ)-producing T cells, the phenotypes of recruited T cells, and the granulomatous responses were also similar between WT and TLR2KO immune mice. Together, the findings from this study indicate that TLR2 signaling does not influence memory immunity to M. tuberculosis.Mycobacterium tuberculosis expresses a large repertoire of lipoproteins that can trigger signaling from Toll-like receptor 2 (TLR2) (13), including the 19-kDa lipoprotein (LpqH) (5), LprA (Rv1270) (29), and LprG (Rv1411c) (11). In addition to lipoproteins, lipomannan (31) and phosphatidyl-myo-inositol mannoside (PIM) (12, 16) also interact with TLR2 to initiate cellular activation (16). Despite this collection of TLR2 agonists on the tuberculosis (TB) bacillus, murine studies indicate that TLR2 is not essential for host resistance against acute M. tuberculosis infection (34, 37).It is well appreciated that memory immunity in tuberculosis does not provide long-term protective immunity, as evidenced in humans and experimental infections of mice. In a study performed in Cape Town, South Africa, it was determined that the incidence rate of TB attributable to reinfection after successful chemotherapy was four times that of new TB (40). In mouse models, immunological memory induced by M. tuberculosis infection can provide short-term protection, as evidenced by early reduction in the bacterial burden in the lungs following reexposure (6, 36). The memory immune mice exhibit a transient early induction of Th1 cells compared to naïve mice and concomitant early control of bacterial replication. However, despite the skewed kinetics, the memory mice do not achieve bacterial sterility in the lung, and bacteria continue to be maintained in a stable state. Clearly, this major gap in our understanding of how to induce sterilizing memory immunity in TB is an impediment to vaccine development.In vitro studies have documented opposing outcomes from antigen-presenting cells (APC) following interaction of their surface TLR2 with M. tuberculosis. For example, TLR2 signals upregulate B7 expression, induce interleukin 12 (IL-12) secretion (15), and initiate antimicrobial responses within M. tuberculosis-infected macrophages (22). TLR2 also initiates signaling that inhibits major histocompatibility complex (MHC) class II-dependent antigen presentation (24, 26) by macrophages and responsiveness to gamma interferon (IFN-γ) (2, 8, 18, 27). How these opposing changes to APC by TLR2 signals affect naïve T cell differentiation into effector and memory T cells following M. tuberculosis infection remains unclear. Furthermore, recent studies indicate that high expression levels of IL-12 in the environment promote effector T cell development while low expression levels (17) or even the absence (28, 41) of the cytokine is favorable for central memory T cell development. Together with the report that TLR2 regulates IL-10 production from macrophages and dendritic cells (DC) following M. tuberculosis infection (15, 30), these findings suggest that TLR2 signals may modulate the inflammatory milieu during T cell priming to influence effector versus memory T cell development.The literature on inhibition of APC function by TLR2 predicts that removal of TLR2 may improve APC functions and lead to better memory immunity. On the other hand, the finding that TLR2 signaling is anti-inflammatory and consequently conducive to memory T cell development predicts that absence of TLR2 may result in poorer memory immunity. Therefore, in this study, we examined whether the absence of TLR2 improved or worsened the capacity of the host to generate memory immunity upon rechallenge with M. tuberculosis.