Preferential positive selection of T lymphocytes which express two different TCR alpha chains, an endogenous and a transgenic

Multiple sclerosis: autoimmunity and viruses

PURPOSE OF REVIEW

This review will explore two new aspects of the involvement of viruses in multiple sclerosis pathogenesis. The first aspect is the complex interactions between viruses. The second aspect is the proposal of a mechanism by which autoreactive T cells are able to escape thymic selection and potentially recognize self and a pathogen.

RECENT FINDINGS

With regard to viruses, recent work has demonstrated that one virus may enhance the replication of another virus, potentially leading to an increase in inflammation and disease progression. Also, interactions between human endogenous retroviruses, which likely do not replicate, and certain herpes viruses, may also play a role in disease pathogenesis. Mechanistically, T cells expressing dual T-cell receptors would be able to recognize self and a foreign antigen specifically. Therefore, human endogenous retroviruses potentially play a role in multiple sclerosis pathogenesis, and both interactions between multiple viruses and autoreactive CD8(+) T cells with dual T-cell receptors may play a role in the pathogenesis of the disease.

SUMMARY

The complex interactions between multiple viral infections, either within the central nervous system or in the periphery, and the host immune response to viral infection may be such that a variety of viral specificities result in the activation of T cells that recognize self and induce multiple sclerosis. Therefore, it is unlikely that any one microbe will be determined to be the causative agent of multiple sclerosis as reflected by the number of potential triggering mechanisms of the disease.

Chaperone-assisted thermostability engineering of a soluble T cell receptor using phage display

We here report a novel phage display selection strategy enabling fast and easy selection of thermostabilized proteins. The approach is illustrated with stabilization of an aggregation-prone soluble single chain T cell receptor (scTCR) characteristic of the murine MOPC315 myeloma model. Random mutation scTCR phage libraries were prepared in E. coli over-expressing the periplasmic chaperone FkpA, and such over-expression during library preparation proved crucial for successful downstream selection. The thermostabilized scTCR^mut variants selected were produced in high yields and isolated as monomers. Thus, the purified scTCRs could be studied with regard to specificity and equilibrium binding kinetics to pMHC using surface plasmon resonance (SPR). The results demonstrate a difference in affinity for pMHCs that display germ line or tumor-specific peptides which explains the tumor-specific reactivity of the TCR. This FkpA-assisted thermostabilization strategy extends the utility of recombinant TCRs and furthermore, may be of general use for efficient evolution of proteins.

Functional phage display of two murine alpha/beta T-cell receptors is strongly dependent on fusion format, mode and periplasmic folding assistance

Phage display has been instrumental for the success of antibody (Ab) technology. The aim of the present study was to explore phage display of soluble T-cell receptors (TCRs). A library platform that supports engineering and selection of improved TCRs to be used as detection reagents for specific antigen presentation will be very useful. In such applications, high, equal and clone independent display levels are a prerequisite for 'fair' selection. Therefore, we explored how different pIII fusion formats and modes affected the display levels of two murine alpha/beta TCRs. Both are derived from T-cell clones associated with the MOPC315 myeloma model. The results show that the design of the pIII fusion particle significantly affects the subsequent display levels. Furthermore, successful display may be obtained both in phagemid and phage versions. Importantly, improvement of poor display can be achieved by over-expressing the periplasmic chaperone FkpA.

Anti-Class II Antibodies, but not Cytotoxic T-Lymphocyte Antigen 4-Immunoglobulin Hybrid Molecules, Prevent Rejection of Major Histocompatibility Complex Class II-Negative Myeloma in T-Cell Receptor-Transgenic Mice

We have previously shown that tumour-specific CD4+ T cells protect against subcutaneous injections of major histocompatibility complex (MHC) class II-negative MOPC315 myeloma cells. Here, we have interfered with the immunologic events that lead to successful rejection of MOPC315 challenges in T-cell receptor (TCR)-transgenic mice. The CD4+ T cells have a transgene-encoded TCR specific for a MOPC315 V-region idiotypic (Id) peptide presented on the MHC class II molecule E(d). A side-by-side comparison indicated that DNA-recombination-deficient TCR-transgenic mice were better protected against MOPC315 tumour development than recombination-sufficient counterparts, suggesting that B cells or endogenous TCR chains might facilitate tumour progression in this model. Intraperitoneal injections of E(d)-specific antibodies over a period of initial 24 days, abrogated protection against tumours in both strains of mice. By contrast, injections of anticostimulatory molecules (cytotoxic T-lymphocyte antigen 4-immunoglobulin hybrid molecules) had no effect. The findings demonstrate that tumour rejection depends on the presence of MHC class II molecules, despite the fact that MOPC315 tumour cells themselves do not express them. The results are consistent with the idea that secreted myeloma protein is processed and presented by class II+ antigen-presenting cells to Id-specific naïve CD4+ T cells that become activated and kill the myeloma cells by a bystander mechanism. While Id presentation on class II molecules is absolutely required for tumour rejection, costimulatory CD80/CD86 molecules might be dispensible in this process.

Phagocytic dendritic cells from myelomas activate tumor-specific T cells at a single cell level

Antigen-presenting cells (APCs) from subcutaneous mouse MOPC315 plasmacytoma phagocytosed immunoglobulin G-coated magnetic beads, enabling efficient isolation within 2 hours by magnetic separation (APC-MB). Cell morphology was heterogeneous, with some of the cells having dendrites. The surface phenotype of purified tumor APCs-MB was CD11b(+), CD11c(+), CD40(+), CD80(+), CD86(+), and MHC class II(+). Tumor APCs-MB expressed messenger RNA for fractalkine and ABCD-1 chemokines, and for CC-type chemokine receptors CCR5 and CCR7, indicating the presence of mature dendritic cells (DCs). Visualized at a single cell level within 4 hours after disruption of the tumor, APCs-MB induced rapid Ca(++) mobilization in MHC class II-restricted tumor idiotype (Id)-specific cloned CD4(+) T cells. In long-term assays, tumor APCs-MB induced proliferation of naive T cells from Id-specific T-cell receptor transgenic mice. The results suggest that tumor APCs-MB represent a heterogeneous cell population that includes myeloid-derived DCs of various stages of maturation. A considerable fraction (> or = 15%) of DCs is spontaneously primed with tumor-specific antigen.

Dendritic cells purified from myeloma are primed with tumor-specific antigen (idiotype) and activate CD4+ T cells

Multiple myelomas produce tumor-specific antigen (TSA) in the form of idiotype (Id) on monoclonal Ig. CD4(+) T cells can recognize Id-peptide on MHC class II molecules and protect against challenges with MOPC315 cells, which are, as common for myelomas, class II-negative. The present study explains these previous results by demonstrating that Id can be transferred from myeloma cells to antigen-presenting cells (APC), which present processed Id-peptide on their class II molecules to Id-specific T cell receptor-transgenic (TCR-TG) CD4(+) T cells. Id-primed tumor APC were heterogeneous, the majority being dendritic cells with class II(+), CD11b(+) CD11c(+) CD40(+) CD80(+) CD86(+) markers. The APC were localized beneath CD31(+) endothelial cells of tumor microvessels, and their frequency declined with tumor progression. The APC could stimulate Id-specific naive TCR-TG, short-term polarized TCR-TG, and cloned CD4(+) T cells to proliferate and produce cytokines in vitro. Furthermore, small MOPC315 tumors established in Id-specific TCR-TG mice contained clusters of activated (CD69(+)CD25(+)) and proliferating (BrdUrd(+)) Id-specific transgenic CD4(+) blasts. The activated Id-specific T cells were located adjacent to Id-primed dendritic cells in the tumor. Thus, a TSA can be transferred in vivo from myeloma, and possibly other types of cancer cells to APC for MHC class II presentation to CD4(+) T cells.

Resting small B cells present endogenous immunoglobulin variable-region determinants to idiotope-specific CD4(+) T cells in vivo

Antigenic determinants localized within the highly diversified V-regions of Ig are called idiotopes (Id). Processed Id-peptides can be presented on MHC class II molecules to CD4(+) T cells. If B cells present their endogenous Id-peptides, T cell activation could occur in the absence of nominal antigen, a potentially important process in T-B cooperation and immune regulation. To test this idea, we used mice made transgenic for a lambda2 L-chain (Id(+) mice). Another transgenic mouse strain expresses TCR transgenes with specificity for the Id (lambda2), presented on MHC class II molecules. When highly purified sorted Id(+) B cells and Id-specific T cells were sequentially injected into MHC syngeneic SCID host, T cell became blastoid, CD69(+) and proliferated. To exclude any role of host APC, MHC incompatible Rag2(- / -) mice (H-2(b)) were used as recipients for the Id(+) B and Id-specific T cells, with similar results. Exposure to extracellular Id(+) immunoglobulin (Ig) was not sufficient for Id priming of B cells in vivo, highlighting the preferential presentation of Id peptides derived from endogenous Ig, by B cells. The results suggest that B cells presenting Id self-peptides generated by V(D)J recombinations or somatic mutations may directly stimulate T cell in vivo in the absence of conventional antigen.

Development and function of autospecific dual TCR+ T lymphocytes

Recent studies have challenged the long held concept that each T lymphocyte expresses on its surface only a single, unique alphabetaTCR. Dual TCR+ T cells have been recognized, however, their origin and potential to escape screening for self-reactivity remain obscure. We now report the thymic generation of dual alphabetaTCR+ T cells in the H-2Db/H-Y-specific TCR transgenic (Tg) mouse. Dual TCR+ thymocytes were positively selected less efficiently than single TCR+ thymocytes, although a subset attained maturity. Importantly, when TCR Tg mice were bred onto a negatively selecting background, auto-specific cells survived central deletion and matured as CD4+ dual TCR+ cells. These cells were autoreactive when CD8 expression was restored. The existence of autospecific, dual TCR+ T cells may have implications for the maintenance of self tolerance.

Immunoglobulin as a vehicle for foreign antigenic peptides immunogenic to T cells

Antibody (Ab) molecules may serve as targeting vehicles for delivery of foreign antigenic peptides to antigen presenting cells (APC). An attractive strategy is to substitute segments between beta-strands of immunoglobulin (Ig) constant (C)-region domains with antigenic peptides. For this to work, the mutant Ab must maintain its conformation so that it can be secreted from transfected cells. Furthermore, the antigenic peptides must be excised by the processing machinery of APC and loaded onto major histo-compatibility complex (MHC) class II molecules. To test this, we have introduced a peptide of eleven amino acids (a.a.) as either of three different loops in the first C-region domain of the heavy (H) chain (CH1) of human IgG3. When the resulting mutant H chain genes were expressed in a fibroblast cell line equipped with proper class II molecules, the H chains were retained intracellularly, probably due to the light (L) chain deficiency of the fibroblasts. Nevertheless, by the endogenous class II processing pathway, presentation of the epitope to CD4+ cells was observed for all three mutants. The presentation efficiency, however, depended on the position of the peptide in the H chain. This could be due to influence of flanking sequences, which differ in the three loop replacement mutants. When L chain-expressing Chinese hamster ovary (CHO) lambda cells were transfected with the same constructs, two out of the three mutant Ig were secreted. The mutants had the expected antigen specificity and were recognized by anti-IgG Ab. When added exogenously to dendritic cell APC, the mutant IgG3 were processed, and the liberated foreign epitopes presented to T cells. The results suggest that the loops connecting beta-strands in the Ig fold may be replaced by foreign peptides, which upon processing become stimulatory to CD4+ T cells. Combined with the well-known targeting function of antibodies, this principle may be useful for construction of a new generation of vaccines.

A range of CD4 T cell tolerance: partial inactivation to organ-specific antigen allows nondestructive thyroiditis or insulitis

T cell receptor (TCR) transgenic mice specific for hen egg lysozyme (HEL) were crossed with mice expressing HEL on the thyroid epithelium, on pancreatic islet beta cells, or systemically. Depending on the pattern of HEL expression, deletion of double-positive thymocytes ranged from minimal to complete, and peripheral CD4 cells exhibited graded reduction in TCR expression, in vitro responsiveness, and in vivo helper ability. CD4 cells were least tolerant in TCR/thyroid-HEL and TCR/islet-HEL mice, which developed an extensive lymphocytic thyroiditis or insulitis that nevertheless did not eliminate HEL-expressing endocrine cells. Autoreactive CD4 clones thus escape the thymus under a range of circumstances, retain sufficient function to initiate subclinical autoimmune inflammation when self-antigens are concentrated in the thyroid or pancreas, and may regulate progression of subclinical inflammation to destructive autoimmune disease.

Dual T cell receptor T cells have a decreased sensitivity to physiological ligands due to reduced density of each T cell receptor

A considerable fraction of T cells express two distinct T cell receptors (TCR), mainly due to expression of two TCR alpha chains. It has been suggested that such dual-TCR cells could have a role in autoimmunity. However, as such cells express less of each TCR, they could be less sensitive to their physiological ligand, i.e. peptide plus major histocompatibility complex molecules (MHC). We tested this hypothesis in a transgenic TCR model in which most T cells express different amounts of the transgene-encoded TCR, due to expression of endogenous TCR alpha chains. Five Th1 clones derived from lambda2(315) immunoglobulin light chain-specific TCR-transgenic mice expressed different levels of the transgene-encoded TCR, ranging from approximately 10,000 to approximately 50,000 TCR per cell. Cytosolic Ca2+ mobilization in single T cells from these clones elicited by lambda2(315) peptide-pulsed, I-Ed-expressing antigen-presenting cells, correlated linearly with the relative transgene-encoded TCR expression. The peptide requirement for half-maximal T cell proliferation showed a similar correlation, with low TCR levels requiring higher peptide concentration. Corroborative evidence was obtained by deployment of short-term polyclonal CD4+ lines from TCR-transgenic mice. Such lines had reduced early (Ca2+ mobilization) and late (lymphokine and proliferation) responses, compared with T cell lines from recombination-deficient TCR-transgenic severe combined immunodeficiency mice (which express only a single transgene-encoded TCR). Taken together, the Ca2+ responses increase gradually with increasing TCR expression per cell, similar to the previously described analog Ca2+ signaling elicited by increasing amounts of peptide/MHC [Røtnes et al., Eur. J. Immunol. 1994. 24: 851]. Surprisingly small reductions in TCR expression per cell reduce T cell responsiveness. This suggests that dual-TCR T cells are immunologically less effective than single-TCR T cells.

Peripheral T cell tolerance as a tumor escape mechanism: deletion of CD4+ T cells specific for a monoclonal immunoglobulin idiotype secreted by a plasmacytoma

Tumors could escape an immune attack by inducing peripheral T cell tolerance. To test this, T cell receptor (TCR)-transgenic mice were injected with plasmacytoma cells secreting a highly tumor-specific antigen, a monoclonal immunoglobulin (Ig), for which the transgene-encoded TCR is specific. The TCR recognizes a third hypervariable region idiotypic (Id) peptide of the Ig, presented by a class II molecule on host antigen-presenting cells. The TCR-transgenic mice have previously been shown to be protected against an Id+ plasmacytoma challenge. In the present experiments, the protection was deliberately overwhelmed by subcutaneous injection of large numbers of plasmacytoma cells. Such tumor mice, chronically exposed to increasing amounts of monoclonal Ig, delete Id-specific CD4+ T cells in their peripheral lymphoid organs and in the tumor. The residual CD4+ cells express endogenous, rather than transgene-encoded TCR alpha chains. Peripheral deletion, functional T cells unresponsiveness, and thymocyte deletion are all first detected at the same serum concentration of monoclonal Ig, approximately 50 micrograms/ml (0.3 microM), and become more and more profound as the tumor burden increases. The results suggest that peripheral T cell tolerance to Id could be a tumor escape mechanism in patients with B cell malignancies. In addition, the findings have implications for T cell tolerance to Ig V regions in normal individuals.