Major histocompatibility complex class II (MHC-IIExosomes are 50-to 80-nm membrane vesicles that are released by many cell types, including reticulocytes, B cells, and dendritic cells (DCs) (16, 17, 33-35, 40, 42, 49, 53). Invagination of the limiting membrane of late endosomes leads to the formation of intraluminal vesicles in multivesicular endosomes. The intraluminal vesicles are secreted as exosomes upon the fusion of multivesicular endosomes with the plasma membrane.Exosomes from B cells contain major histocompatibility complex class II (MHC-II) molecules and can stimulate CD4 ϩ T-cell responses in vitro (40), although they may be more capable of activating primed T cells than naïve CD4 ϩ T cells (27). The activation of naïve CD4 ϩ T cells by DC exosomes occurs via an indirect pathway in which the exosomes and their constituent peptide-MHC-II molecules are presented in the context of intact antigen (Ag)-presenting cells (APCs) (e.g., DCs that may be MHC-II negative but must bear the costimulatory molecules CD80 and CD86 [48]). The presence of ICAM-1 on exosomes is important for naïve T-cell priming (43).While the shedding of exosomes can be constitutive (27, 40), it can also be significantly enhanced by the stimulation of certain receptors, e.g., Toll-like receptors (TLRs) and the P2X7 purinergic receptor (P2X7R), which trigger inflammatory responses (37, 38). P2X7R can be activated by ATP, which is released into the extracellular milieu following cell death or injury (50). P2X7R signaling induces the assembly of inflammasome signaling complexes (10), which drive the proteolytic activation of caspase-1 and the maturation of interleukin 1b (IL-1b). Another P2X7R-induced response is the rapid extracellular release of MHC-II molecules (38), which was previously observed within 15 min of the addition of ATP and resulted in the release of ϳ15% of the total MHC-II pool in macrophages within 90 min (38). Released MHC-II molecules were contained in two membrane fractions: larger (100-to 1,000-nm) plasma membrane-derived microvesicles and smaller (50-to 80-nm) exosomes. The ATP-stimulated release of MHC-II was markedly reduced in macrophages isolated from NLRP3 knockout or ASC knockout mice. Thus, P2X7R activation of the NLRP3 inflammasome induces the biogenesis and release of MHC-II-containing membranes. The precedent of synergy between lipopolysaccharide (LPS) and ATP suggests that MHC-II shedding might be enhanced in the context of bacterial infection, but this hypothesis has not been explored. Mycobacterium tuberculosis is a major human pathogen that infects one-third of the world population. M. tuberculosis and the related organism Mycobacterium bovis strain BCG infect host cells and regulate host cell functions by signaling through innate immune receptors, including TLR2. Cells infected with M. tuberculosis also secrete exosomes containing mycobacterial molecules that function as PAMPs (pathogen-associated mo-* Corresponding author. Mailing address:

SummarySome pathogenic bacteria form thick capsules that both block immune responses through inhibition of complement deposition and phagocytosis and stimulate a weak response resulting from a lack of T-cell involvement. Contrary to this model, capsular polysaccharides from 23 serotypes of Streptococcus pneumoniae have been successfully used in a multivalent vaccine in the absence of a carrier protein. Furthermore, type I pneumococcal polysaccharide (Sp1) has been shown to activate T cells in vivo and in vitro via an uncharacterized mechanism. In the present report, we demonstrate that Sp1 utilizes the major histocompatibility complex (MHC) class II pathway in antigen-presenting cells (APCs) for processing and presentation. APCs internalize and process Sp1 through a nitric oxidedependent mechanism and, once inside the cell, it associates with MHC II proteins in an H-2M-dependent manner that leads to in vivo T-cell activation. These results establish that Sp1 activates T cells which can lead to abscess formation in mice through an H-2M-dependent polysaccharide antigen presentation pathway in APCs, potentially contributing to pneumococcal polysaccharide vaccine efficacy through the recruitment of T-cell help.

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