IntroductionUnlike peptide antigens, which are presented to conventional T cells via major histocompatibility complex (MHC) molecules, lipid antigens are presented to T cells by the MHC-like molecule, CD1. Humans express several nonpolymorphic CD1 molecules, including CD1d, which presents lipids to a unique subset of T cells termed natural killer T (NKT) cells. NKT cells are innate-like lymphocytes defined by their characteristic semi-invariant T-cell receptor that recognizes the potent glycolipid antigen ␣-galactosylceramide (␣GalCer). In addition to this nonphysiologic antigen, NKT cells have recently been shown to respond to exogenous bacterial-derived lipid antigens 1-3 as well as endogenous lipids presented by antigen-presenting cells (APCs) responding to innate stimuli. 2,4,5 Resulting in part from their inherent memory phenotype and rapid cytokine secretion after stimulation, NKT cells complement innate signals and promote adaptive immunity. 6 ␣GalCer has been shown to have adjuvant-like properties that enhance T-dependent and T-independent humoral immune responses. [7][8][9][10] To generate antibodies to T-dependent antigens, B cells require cognate T-cell help from conventional T helper (Th) cells that recognize the same antigen or antigenic complex, which has been internalized by the B-cell receptor (BCR). ␣GalCer has been shown to boost dendritic cell (DC)-Th interactions that augment Th priming and indirectly promote B-cell help. 11 However, B cells also express CD1d and have been shown to present lipid antigens and recruit cognate help from NKT cells. 8,9,12,13 Recent work has shown that BCR-mediated uptake of model B-cell antigens linked to ␣GalCer elicits cognate NKT help for lipid-specific B cell responses in vivo and in vitro. [12][13][14] However, B cells also internalize and present lipid antigens to NKT cells by pathways that are independent of the BCR, 15 and the mechanisms by which this occurs are unknown. Our previous studies have shown that DCs use the low-density lipoprotein (LDL)-receptor pathway to endocytose apolipoprotein E (apoE)-bound lipid antigens for subsequent presentation to NKT cells. 16 ApoE, found in serum very low density lipoproteins or secreted locally by DCs and macrophages, rapidly binds exogenous lipid antigens. ApoE-lipid antigen complexes are efficiently captured by the LDL-R on DCs and delivered to intracellular CD1d where the lipid antigen is loaded. Whereas DCs use multiple nonspecific pathways of antigen uptake, B-cell antigen uptake is thought to be restricted to the BCR. An LDL-R-mediated pathway in B cells would provide a means for innate help by NKT cells stimulating polyclonal B-cell activation. We thus investigated whether B cells may also use an apoEmediated pathway for lipid antigen presentation. Methods NKT cellsTwo CD1d-restricted human NKT cell clones (BM2a.3 and J3N.5, previously described 17 ) and 1 CD1d-restricted human NKT line (M0) were used in these studies, and similar results were obtained. M0 was derived by successive rounds of ␣GalCer-CD1d-t...

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