Immune responses are initiated and primed by dendritic cells (DCs) that cross-present exogenous antigen. The CD74 (invariant chain) chaperone protein is thought to exclusively promote DC priming in the context of MHC class II. However, we demonstrate herein a CD74-dependent MHC class I cross-presentation pathway in DCs that plays a major role in the generation of MHC class I restricted, cytolytic T lymphocyte (CTL) responses against viral protein- and cell-associated antigens. CD74 associates with MHC class I molecules in the endoplasmic reticulum of DCs and mediates trafficking of MHC class I to endolysosomal compartments for loading with exogenous peptides. We conclude that CD74 plays a hitherto, undiscovered physiological function in endolysosomal DC cross-presentation for priming MHC class I-mediated CTL responses.

Signal transduction in Mycobacterium tuberculosis is mediated primarily by the Ser/Thr protein kinases and the two-component systems. The Ser/Thr kinase PknH has been shown to regulate growth of M. tuberculosis in a mouse model and in response to NO stress in vitro. Comparison of a pknH deletion mutant (⌬pknH) with its parental M. tuberculosis H37Rv strain using iTRAQ enabled us to quantify >700 mycobacterial proteins. Among these, members of the hypoxia-and NO-inducible dormancy (DosR) regulon were disregulated in the ⌬pknH mutant. Mycobacterium tuberculosis, the causative agent of tuberculosis, is a human intracellular pathogen that is phagocytosed by alveolar macrophages and subsequently "walled off" by the host immune response within granulomas (1). M. tuberculosis is able to persist within the hostile microenvironment of the granuloma, which is thought to include hypoxic, acidic, and nutrient-poor conditions and immune effectors such as nitric oxide (NO) 5 (2). The survival and persistence of M. tuberculosis in this environment requires the ability to sense external signals and mount an effective adaptive response. M. tuberculosis possesses multiple families of signal transduction systems, including the Ser/Thr protein kinases (STPKs) and the two-component regulatory systems (TCSs) (3).In a previous study, we found that the STPK PknH functions as an in vivo growth regulator (4). Hypervirulence was consistently detected in BALB/c mice infected with a pknH deletion mutant in M. tuberculosis after 3-4 weeks of infection (4), corresponding to the onset of adaptive immunity. Therefore, we hypothesized that M. tuberculosis uses the PknH kinase-mediated pathways to respond to host-induced signals to regulate its in vivo growth. Nitric oxide produced by the inducible nitricoxide synthase of the host macrophages plays a key role in controlling bacillary growth during the chronic phase of infection following activation of the host immune response (5). In vitro experiments revealed that the ⌬pknH mutant is more resistant to NO compared with WT (4), indicating that PknH may act as a sensor of NO to regulate M. tuberculosis growth in vivo.Predictions from bioinformatics analysis and studies using in vitro kinase assays have identified three endogenous substrates of PknH kinase: EmbR (6), a transcriptional regulator of the embCAB genes involved in lipoarabinomannan and arabinogalactan synthesis; DacB1, a cell division-related protein; and Rv0681, a putative transcriptional regulator (7). However, the substrates and downstream effectors of PknH signaling in response to NO stimulus have yet to be discovered.The DosR system, also known as DevR, is one of 11 pairs of TCSs present in M. tuberculosis (3). It is well established that DosR responds to hypoxia, NO, and CO via signaling through two cognate sensor kinases, DosS (DevS) and DosT (8,9) to activate transcription of a defined set of ϳ50 genes termed the "dormancy" or DosR regulon (10). Genes belonging to the DosR regulon, including dosR, are up-regulated in the Wayne m...