It is likely that human genetic differences mediate susceptibility to viral infection and virus-triggered disorders. OAS genes encoding the antiviral enzyme 2',5'-oligoadenylate synthetase (2'5'AS) are critical components of the innate immune response to viruses. This enzyme uses adenosine triphosphate in 2'-specific nucleotidyl transfer reactions to synthesize 2',5'-oligoadenylates, which activate latent ribonuclease, resulting in degradation of viral RNA and inhibition of virus replication. We showed elsewhere that constitutive (basal) activity of 2'5'AS is correlated with virus-stimulated activity. In the present study, we asked whether constitutive activity is genetically determined and, if so, by which variants. Analysis of 83 families containing two parents and two children demonstrated significant correlations between basal activity in parent-child pairs (P<.0001) and sibling pairs (P=.0044), but not spousal pairs, suggesting strong genetic control of basal activity. We next analyzed association between basal activity and 15 markers across the OAS gene cluster. Significant association was detected at multiple markers, the strongest being at an A/G single-nucleotide polymorphism at the exon 7 splice-acceptor site (AG or AA) of the OAS1 gene. At this unusual polymorphism, allele G had a higher gene frequency in persons with high enzyme activity than in those with low enzyme activity (0.44 vs. 0.20; P=3 x 10(-11)). Enzyme activity varied in a dose-dependent manner across the GG, GA, and AA genotypes (tested by analysis of variance; P=1 x 10(-14)). Allele G generates the previously described p46 enzyme isoform, whereas allele A ablates the splice site and generates a dual-function antiviral/proapoptotic p48 isoform and a novel p52 isoform. This genetic polymorphism makes OAS1 an excellent candidate for a human gene that influences host susceptibility to viral infection.

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...

B cell activation and Ab production in response to protein Ags requires presentation of peptides for recruitment of T cell help. We and others have recently demonstrated that B cells can also acquire innate help by presenting lipid Ags via CD1d to NKT cells. Given the newfound contribution of NKT cells to humoral immunity, we sought to identify the pathways that regulate CD1 molecule expression in human B cells. We show that ex vivo, activated and memory B cells expressed lower levels of CD1d compared with resting, naive, and marginal zone-like B cells. In vitro, CD1d was downregulated by all forms of B cell activation, leaving a narrow temporal window in which B cells could activate NKT cells. CD1c expression and function also decreased following activation by CD40L alone, whereas activation via the BCR significantly upregulated CD1c, particularly on marginal zone-like B cells. We found that the CD40L-induced downreglation of CD1d and CD1c correlated with diminished expression of retinoic acid receptor α (RARα) response genes, an effect that was reversed by RARα agonists. However, BCR-induced upregulation of CD1c was independent of the RAR pathway. Our findings that both CD1d and CD1c are upregulated by RARα signaling in human B cells is distinct from effects reported in dendritic cells, in which CD1c is inversely downregulated. One functional consequence of CD1d upregulation by retinoic acid was NKT cell cytotoxicity toward B cells. These results are central to our understanding of how CD1-restricted T cells may control humoral immunity.