Proteolytic processing is required for viral superantigen activity

A defective viral superantigen-presenting phenotype in HLA-DR transfectants is corrected by CIITA

Activation of T lymphocytes by mouse mammary tumor virus superantigen (vSAg) requires binding to MHC class II molecules. The subcellular location where functional interactions occur between MHC class II molecules and vSAgs is still a matter of debate. To gain further insight into this issue, we have used human epithelial HeLa cells expressing HLA-DR1. Surprisingly, the human cells were unable to present transfected vSAg7 or vSAg9 to a series of murine T cell hybridomas. The defect is not related to a lack of vSAg processing, because these cells can indirectly activate T cells after coculture in the presence of B lymphocytes. However, after IFN-gamma treatment, the HeLa DR1(+) cells became apt at directly presenting the vSAg. Furthermore, transfection of CIITA was sufficient to restore presentation. Reconstitution experiments demonstrated the necessity of coexpressing HLA-DM and invariant chain (Ii) for efficient vSAg presentation. Interestingly, inclusion of a dileucine motif in the DRbeta cytoplasmic tail bypassed the need for HLA-DM expression and allowed the efficient presentation of vSAg7 in the presence of Ii. A similar trafficking signal was included in vSAg7 by replacing its cytoplasmic tail with the one of Ii. However, sorting of this chimeric Ii/vSAg molecule to the endocytic pathway completely abolished both its indirect and direct presentation. Together, our results suggest that functional vSAgs-DR complexes form after the very late stages of class II maturation, most probably at the cell surface.

Relaxed DM requirements during class II peptide loading and CD4+ T cell maturation in BALB/c mice

Current ideas about DM actions have been strongly influenced by studies of mutant strains expressing the H-2(b) haplotype. To evaluate DM contributions to class II activities in BALB/c mice, we generated a novel mutation at the DMa locus via embryonic stem cell technology. Unlike long-lived A(b)/class II-associated invariant chain-derived peptide (CLIP) complexes, mature A(d) and E(d) molecules are loosely occupied by class II-associated invariant chain-derived peptide and are SDS unstable. BALB/c DM mutants weakly express BP107 conformational epitopes and toxic shock syndrome toxin-1 superantigen-binding capabilities, consistent with partial occupancy by wild-type ligands. Near normal numbers of mature CD4(+) T cells fail to undergo superantigen-mediated negative selection, as judged by TCR Vbeta usage. Ag presentation assays reveal consistent differences for A(d)- and E(d)-restricted T cells. Indeed, the mutation leads to decreased peptide capture by A(d) molecules, and in striking contrast causes enhanced peptide loading by E(d) molecules. Thus, DM requirements differ for class II structural variants coexpressed under physiological conditions in the intact animal.

A short sequence in the N-terminal region is required for the trimerization of type XIII collagen and is conserved in other collagenous transmembrane proteins

The recombinant transmembrane protein type XIII collagen is shown to reside on the plasma membrane of insect cells in a 'type II' orientation. Expressions of deletion constructs showed that sequences important for the association of three alpha1(XIII) chains reside in their N- rather than C-terminal portion. In particular, a deletion of residues 63-83 immediately adjacent to the transmembrane domain abolished the formation of disulfide-bonded trimers. The results imply that nucleation of the type XIII collagen triple helix occurs at the N-terminal region and that triple helix formation proceeds from the N- to the C-terminus, in opposite orientation to that of the fibrillar collagens. Interestingly, a sequence homologous to the deleted residues was found at the same plasma membrane-adjacent location in other collagenous transmembrane proteins, suggesting that it may be a conserved association domain. The type XIII collagen was secreted into insect cell medium in low amounts, but this secretion was markedly enhanced when the cytosolic portion was lacking. The cleavage occurred in the non-collagenous NC1 domain after four arginines and was inhibited by a furin protease inhibitor.

A novel baculovirus envelope fusion protein with a proprotein convertase cleavage site

The entry mechanism of Spodoptera exigua multicapsid nucleopolyhedrovirus (SeMNPV), a group II NPV, in cultured cells was examined. SeMNPV budded virus (BV) enters by endocytosis as do the BVs of the group I NPVs, Autographa californica (Ac) MNPV and Orgyia pseudotsugata (Op) MNPV. In group I NPVs, upon infection acidification of the endosome triggers fusion of the viral and endosomal membrane, which is mediated by the BV envelope glycoprotein GP64. However, the SeMNPV genome lacks a homolog of GP64 envelope fusion protein (EFP). A functional homolog of the OpMNPV GP64 EFP was identified in SeMNPV ORF8 (Se8; 76 kDa) and appeared to be the major BV envelope protein. Surprisingly, a 60-kDa cleavage product of this protein is present in the BV envelope. A furin-like proprotein convertase cleavage site (R-X-K/R-R) was identified immediately upstream of the N-terminus of the mature Se8 protein and this site was also conserved in the Lymantria dispar (Ld) MNPV homolog (Ld130) of Se8. Syncytium formation assays showed that Se8 and Ld130 alone were sufficient to mediate membrane fusion upon acidification of the medium. Furthermore, C-terminal GFP-fusion proteins of Se8 and Ld130 were primarily localized in the plasma membrane of insect cells. This is consistent with their fusogenic activity and supports the conclusion that the Se8 gene product is a functional homolog of the GP64 EFP.

Intercellular transfer of a soluble viral superantigen

Mouse mammary tumor virus (MMTV) superantigens (vSAgs) can undergo intercellular transfer in vivo and in vitro such that a vSAg can be presented to T cells by major histocompatibility complex (MHC) class II proteins on antigen-presenting cells (APCs) that do not express the superantigen. This process may allow T-cell activation to occur prior to viral infection. Consistent with these findings, vSAg produced by Chinese hamster ovary (CHO) cells was readily transferred to class II IE and IA (H-2(k) and H-2(d)) proteins on a B-cell lymphoma or mouse splenocytes. Fixed class II-expressing acceptor cells were used to demonstrate that the vSAg, but not the class II proteins, underwent intercellular transfer, indicating that vSAg binding to class II MHC could occur directly at the cell surface. Intercellular transfer also occurred efficiently to splenocytes from endogenous retrovirus-free mice, indicating that other proviral proteins were not involved. Presentation of vSAg7 produced by a class II-negative, furin protease-deficient CHO variant (FD11) was unsuccessful, indicating that proteolytic processing was a requisite event and that proteolytic activity could not be provided by an endoprotease on the acceptor APC. Furthermore, vSAg presentation was effected using cell-free supernatant from class II-negative, vSAg-positive cells, indicating that a soluble molecule, most likely produced by proteolytic processing, was sufficient to stimulate T cells. Because the membrane-proximal endoproteolytic cleavage site in the vSAg (residues 68 to 71) was not necessary for intercellular transfer, the data support the notion that the carboxy-terminal endoproteolytic cleavage product is an active vSAg moiety.

Alternative proteolytic processing of mouse mammary tumor virus superantigens

Mouse mammary tumor viruses express a superantigen essential for their life cycle. It has been proposed that viral superantigens (vSags) require processing by prohormone convertases (PCs) for activity. We now observe, using a panel of mutant forms of potential PC cleavage sites and in vitro cleavage assays, that only the CS1 (position 68 to 71) and CS2 (position 169 to 172) sites are utilized by furin and PC5. Other members of the convertase family that are expressed in lymphocytes are not endowed with this activity. Furthermore, mutant forms of two different viral superantigens, vSag7 and vSag9, which completely abrogated in vitro processing by convertases, were efficient in functional presentation to responsive T-cell hybridomas. This effect was observed in both endogenous presentation and paracrine transfer of the vSag. Processing by convertases thus appears not to be essential for vSag function. Finally, we have identified the purified endosomal protease cathepsin L as another protease that is able to cleave convertase mutant vSag in vitro, yielding fragments similar to those detected in vivo, thus suggesting that proteases other than convertases are involved in the activation of vSags.

Superantigens: mechanisms by which they may induce, exacerbate and control autoimmune diseases

Superantigens are polypeptide molecules produced by a broad range of infectious microorganisms which elicit excessive and toxic T-cell responses in mammalian hosts. In light of this property and the fact that autoimmune diseases are frequently the sequelae of microbial infections, it has been suggested that superantigens may be etiologic agents of autoreactive immunological responses resulting in initiation, exacerbation or relapse of autoimmune diseases. This article relates the biology of superantigens to possible mechanisms by which they may exert these activities and reviews the evidence for their roles in various human and animal models of autoimmune disease. Finally, a mechanism of active suppression by superantigen-activated CD4+ T-cells that could be exploited for therapy as well as prophylaxis of human autoimmune diseases is proposed.

Role of dendritic cells in the immune response induced by mouse mammary tumor virus superantigen

After mouse mammary tumor virus (MMTV) infection, B lymphocytes present a superantigen (Sag) and receive help from the unlimited number of CD4(+) T cells expressing Sag-specific T-cell receptor Vbeta elements. The infected B cells divide and differentiate, similarly to what occurs in classical B-cell responses. The amplification of Sag-reactive T cells can be considered a primary immune response. Since B cells are usually not efficient in the activation of naive T cells, we addressed the question of whether professional antigen-presenting cells such as dendritic cells (DCs) are responsible for T-cell priming. We show here, using MMTV(SIM), a viral isolate which requires major histocompatibility complex class II I-E expression to induce a strong Sag response in vivo, that transgenic mice expressing I-E exclusively on DCs (I-EalphaDC tg) reveal a strong Sag response. This Sag response was dependent on the presence of B cells, as indicated by the absence of stimulation in I-EalphaDC tg mice lacking B cells (I-EalphaDC tg muMT(-/-)), even if these B cells lack I-E expression. Furthermore, the involvement of either residual transgene expression by B cells or transfer of I-E from DCs to B cells was excluded by the use of mixed bone marrow chimeras. Our results indicate that after priming by DCs in the context of I-E, the MMTV(SIM) Sag can be recognized on the surface of B cells in the context of I-A. The most likely physiological relevance of the lowering of the antigen threshold required for T-cell/B-cell collaboration after DC priming is to allow B cells with a low affinity for antigen to receive T-cell help in a primary immune response.

Redundant proteolytic activation of a viral superantigen

Proteolytic activation of viral superantigens (vSAgs)4 expressed in Chinese hamster ovary (CHO) cells is required for T cell stimulation, and is mediated primarily by the protein convertase (PC) furin. Three PC recognition sites are highly conserved in vSAgs, but it was not known which sites are required for PC dependent vSAg activation. Moreover, because the PC recognition sites are not conserved in all functional vSAgs it was possible that activation could occur by processing at any of several sites. To identify the location(s) where processing of vSAg7 generates an active superantigen, each of two PC recognition sites, and a third related site were altered by in vitro mutagenesis, and the mutant proteins were tested for their abilities to activate T cells. Mutation of the PC recognition site at position 68-71 in vSAg7 had no effect on its ability to activate T cells. Mutation of the processing site at position 169-172 completely abolished T cell activation, and indicated that cleavage at this position was obligatory for proteolytic activation of vSAg7. However, introduction of a PC recognition site at position 192-195, a position that in many other vSAgs encodes a PC recognition site, restored activity to a vSAg7 protein that lacked a recognition site at position 169-172. The data revealed that processing of vSAgs at either position 169-172 or 192-195 was sufficient for vSAg7 activation, and explain how vSAgs that lack some PC recognition sites can be activated by proteolytic processing.

Transposon-mediated random insertions and site-directed mutagenesis prevent the trafficking of a mouse mammary tumor virus superantigen

Mouse mammary tumor viruses (MMTVs) encode superantigens (Sags) which are critical to the life cycle of infectious virus and can mediate extensive deletion of T lymphocytes when expressed by endogenous proviruses. Little is known about the structure, intracellular trafficking, or nature of Sag association with major histocompatibility (MHC) class II products. In order to gain a better understanding of Sag structure-function relationships, we extensively mutagenized this type II glycoprotein using two different approaches: transposon-mediated random in-frame insertion mutagenesis and site-directed mutagenesis targeting clusters of charged residues. We find that 31 codon insertions are infrequently tolerated in Mtv-7 Sag, with just 1 of 14 insertion mutants functionally presented on the surface of B cells. Surprisingly, similar effects were observed with Sag mutants with substitutions at pairs of charged residues; only 2 of 6 mutants trafficked to the plasma membrane and stimulated T cells, 1 with a temperature-sensitive phenotype. The data suggest that the nonfunctional Mtv-7 Sag mutants are stringently retained in the endoplasmic reticulum due to conformational defects rather than disrupted interactions with MHC class II, thus identifying charged amino acids critical to the structural stability of viral superantigens.

Mtv-1 superantigen trafficks independently of major histocompatibility complex class II directly to the B-cell surface by the exocytic pathway

Presentation of the Mtv-1 superantigen (vSag1) to specific Vbeta-bearing T cells requires association with major histocompatibility complex class II molecules. The intracellular route by which vSag1 trafficks to the cell surface and the site of vSag1-class II complex assembly in antigen-presenting B lymphocytes have not been determined. Here, we show that vSag1 trafficks independently of class II to the plasma membrane by the exocytic secretory pathway. At the surface of B cells, vSag1 associates primarily with mature peptide-bound class II alphabeta dimers, which are stable in sodium dodecyl sulfate. vSag1 is unstable on the cell surface in the absence of class II, and reagents that alter the surface expression of vSag1 and the conformation of class II molecules affect vSag1 stimulation of superantigen reactive T cells.

Furin: a mammalian subtilisin/Kex2p-like endoprotease involved in processing of a wide variety of precursor proteins

Limited endoproteolysis of inactive precursor proteins at sites marked by paired or multiple basic amino acids is a widespread process by which biologically active peptides and proteins are produced within the secretory pathway in eukaryotic cells. The identification of a novel family of endoproteases homologous with bacterial subtilisins and yeast Kex2p has accelerated progress in understanding the complex mechanisms underlying the production of the bioactive materials. Seven distinct proprotein convertases of this family (furin, PC2, PC1/PC3, PC4, PACE4, PC5/PC6, LPC/PC7/PC8/SPC7) have been identified in mammalian species, some having isoforms generated via alternative splicing. The family has been shown to be responsible for conversion of precursors of peptide hormones, neuropeptides, and many other proteins into their biologically active forms. Furin, the first proprotein convertase to be identified, has been most extensively studied. It has been shown to be expressed in all tissues and cell lines examined and to be mainly localized in the trans-Golgi network, although some proportion of the furin molecules cycle between this compartment and the cell surface. This endoprotease is capable of cleaving precursors of a wide variety of proteins, including growth factors, serum proteins, including proteases of the blood-clotting and complement systems, matrix metalloproteinases, receptors, viral-envelope glycoproteins and bacterial exotoxins, typically at sites marked by the consensus Arg-Xaa-(Lys/Arg)-Arg sequence. The present review covers the structure and function of mammalian subtilisin/Kex2p-like proprotein convertases, focusing on furin (EC 3.4.21.85).

Mouse mammary tumor virus superantigens require N-linked glycosylation for effective presentation to T cells

Mouse mammary tumor viruses (MMTVs) encode superantigens that associate with major histocompatibility complex class II products on antigen-presenting cells and stimulate T cells in a V beta-specific manner. This T cell activation is critical for completion of the viral life cycle and vertical transmission to the next generation. To investigate the functional significance of extensive viral superantigen (Sag) glycosylation, we disrupted the six potential sites for N-linked carbohydrate addition in the Sag encoded by proviral integrant Mtv-1. Shifts in the apparent molecular mass of these mutant glycoproteins suggested that wild-type Mtv-1 Sag is glycosylated on four of its six sites. Intracellular and cell surface staining of the panel of mutants indicated that any decrease in glycosylation resulted in reduced levels of intracellular protein and undetectable surface expression, suggesting that decreased glycosylation leads to rapid Sag degradation and abates trafficking to the plasma membrane. Nevertheless, several mutants with intermediate levels of glycosylation expressed enough Sag on the B cell surface to potently stimulate reactive T cell hybrids. We show there is no specific site bearing N-linked glycosylation that is essential for activity, but at least one carbohydrate addition is necessary for effective B cell presentation of MMTV superantigens to T cells.

Proteolytic processing activates a viral superantigen

Mouse mammary tumor virus (MMTV) superantigens (vSAg) undergo proteolytic processing at residues that have been demonstrated in vitro to be recognition sites for the endoprotease furin. To examine the role of furin in the presentation of vSAg7 to T cells, the vSAg7 and class II MHC IEk genes were introduced into Chinese Hamster Ovary (CHO) cells (furin-positive) and into a furin-negative CHO variant (FD11). Both transfected cell lines efficiently presented peptide antigen and bacterial superantigens to T cell hybridomas. However, while the furin-positive cells presented vSAg7 well, the furin-negative cells presented poorly. Transient transfection of the furin-negative cells with an expression plasmid containing the furin gene restored the ability to present vSAg7 efficiently. The marginal presentation of vSAg7 observed using the furin-negative transfectants was eliminated after culture with the protease inhibitor leupeptin, suggesting that one or more endoproteases other than furin have a detectable but limited capacity to proteolytically activate vSAg7. Biochemical analyses revealed that vSAg7 was largely unprocessed in the absence of furin. Thus, viral superantigens, unlike bacterial superantigens, require proteolytic processing to activate T cells.

Virus-encoded superantigens

Superantigens are microbial agents that have a strong effect on the immune response of the host. Their initial target is the T lymphocyte, but a whole cascade of immunological reactions ensues. It is thought that the microbe engages the immune system of the host to its own advantage, to facilitate persistent infection and/or transmission. In this review, we discuss in detail the structure and function of the superantigen encoded by the murine mammary tumor virus, a B-type retrovirus which is the causative agent of mammary carcinoma. We will also outline what has more recently become known about superantigen activity associated with two human herpesviruses, cytomegalovirus and Epstein-Barr virus. It is likely that we have only uncovered the tip of the iceberg in our discovery of microbial superantigens, and we predict a flood of new information on this topic shortly.

A novel gene product that couples TCR signaling to Fas(CD95) expression in activation-induced cell death

Cross-linking the TCR in T cell hybridomas induces cell apoptosis following activation. This activation-induced apoptosis has been used as a model for clonal deletion of thymocytes or peripheral T cells. Anti-TCR-induced apoptosis of T cell hybridomas requires de novo macromolecular synthesis, including up-regulation of Fas and FasL. The Fas-FasL interaction then activates the apoptosis program. To study apoptosis-specific signaling processes, we generated a mutant T cell hybridoma line defective in induction of apoptosis, but competent to induce activation, upon TCR triggering. Subsequently, we cloned the gene TDAG51, which restored activation-induced apoptosis when transfected into the mutant cell line, and showed that TDAG51 expression was required for Fas expression. Thus, TDAG51 plays an essential role in induction of apoptosis by coupling TCR stimulation to Fas expression.