Processing of DR1-restricted determinants from the fusion protein of measles virus following two distinct pathways

The elucidation of non-classical MHC class II antigen processing through the study of viral antigens

By convention, CD4+ T cells are activated predominantly by Major Histocompatibility Complex class II-bound peptides derived from extracellular (exogenous) antigens. It has been known for decades that alternative sources of antigen, particularly those synthesized within the antigen-presenting cell, can also supply peptides but the impact on TCD4+ responses, sometimes considerable, has only recently become appreciated. This review focuses on the contributions that studies of viral antigen have made to this shift in perspective, concluding with discussions of relevance to rational vaccine design, autoimmunity and cancer immunotherapy.

Mass spectrometry and peptide-based vaccine development

The development of new vaccines against pathogens is an important part of infectious disease control. In the last decade, a variety of proteins giving rise to naturally processed pathogen-derived antigenic peptides, representing B-cell and T-cell epitopes, have been characterized. Numerous candidate vaccines consisting of synthetic peptides are being designed and evaluated, with encouraging results. In this context, the application of mass spectrometry based on the isolation and identification of pathogen-derived peptides from the human leukocyte antigen (HLA) molecules is a major focus of peptide-based vaccine development. Dramatic improvements have been made in mass spectrometer performance for peptide sequencing in terms of increased sensitivity, the ability to rapidly obtain data-directed tandem mass spectra, and the accuracy of mass measurement. This review focuses on the efforts to identify T-cell epitopes for viral and microbial pathogens for directed vaccine development.

Identification of class II HLA-DRB1*03-bound measles virus peptides by 2D-liquid chromatography tandem mass spectrometry

Two-dimensional liquid chromatography (2D-LC), combined with gas phase fractionation tandem mass spectrometry, was used to identify 13 naturally processed peptides originating from measles virus that were presented by HLA-DRB1*03 class II molecules. The peptides are from three of the six measles structural proteins: phosphoprotein, nucleocapsid, and hemagglutinin. These peptides provide an important first step toward understanding the mechanism of immune response to measles virus and development of a new generation of peptide-based vaccines.

Early events of TCR signaling are distinct in human Th1 and Th2 cells

To study the requirements for activation of human Th1 and Th2 cells, soluble peptide/DR1 complexes were prepared from naturally expressed DR1 protein. When immobilized, this material induced T cell activation, as revealed by CD25 up-regulation. Unexpectedly, Th2 cells required a higher density of peptide/DR1 complexes than Th1 cells to initiate CD25 up-regulation. Similar findings were obtained with immobilized or soluble and cross-linked anti-CD3 mAb. In contrast, peptide/DR1 complexes displayed on the surface of nonprofessional APC similarly induced CD25 up-regulation in Th1 and Th2 cells. Signaling events distinguishing human Th1 and Th2 cells following TCR engagement by anti-CD3 mAb were then studied. It was observed that upon TCR triggering, the overall tyrosine phosphorylation profiles were fainter in Th2 than in Th1 clones. Similar results were obtained with Th1- and Th2-polarized polyclonal lines. Varying the dose of anti-CD3 mAb, the kinetics of activation, and coengagement of CD3 and CD28 failed to increase tyrosine phosphorylation in Th2 cells to levels reached in Th1 cells. In contrast, treatment with the tyrosine phosphatase inhibitor phenylarsine oxide resulted in similar tyrosine phosphorylation levels in Th2 and Th1 cells. These findings indicated that Th2 cells had an intrinsically lower TCR-induced tyrosine phosphorylation capacity than Th1 cells, which might be controlled by Th1- and Th2-specific phosphatase profiles. Finally, a weaker association was found between ZAP-70 and CD3zeta in Th2 than in Th1 cells after TCR engagement. Taken together, these results constituted evidence that early events in the TCR signaling cascades are distinct in human Th1 and Th2 cells.

Diversity in MHC class II antigen presentation

Processing exogenous and endogenous proteins for presentation by major histocompatibility complex (MHC) molecules to T cells is the defining function of antigen-presenting cells (APC) as major regulatory cells in the acquired immune response. MHC class II-restricted antigen presentation to CD4 T cells is achieved by an essentially common pathway that is subject to variation with regard to the location and extent of degradation of protein antigens and the site of peptide binding to MHC class II molecules. These subtle variations reveal a surprising flexibility in the ways a diverse peptide repertoire is displayed on the APC surface. This diversity may have profound consequences for the induction of immunity to infection and tumours, as well as autoimmunity and tolerance.

Phagocytosis and protein processing are required for presentation of Cryptococcus neoformans mitogen to T lymphocytes

In addition to eliciting antigen specific T-cell-mediated immunity, Cryptococcus neoformans possesses a mitogen (CnM) that activates naive T cells to proliferate. This mechanism of T-cell activation is accessory cell dependent and major histocompatibility complex unrestricted. CnM-induced T-cell proliferation correlates with internalization of the organism, suggesting that intracellular processing is required to liberate CnM prior to presentation to T cells. To determine whether phagocytosis and processing are required, various inhibitors of accessory cell uptake and processing were used. C. neoformans was observed within the accessory cells. Paraformaldehyde fixation of the accessory cell abrogated presentation of CnM to T cells, indicating that a dynamic accessory cell surface was required. A lysosomotropic agent abrogated the response to CnM but had no effect on a control stimulus that did not require processing. Both aspartic acid and cysteine protease inhibitors blocked effective processing of CnM, so that it was unable to stimulate T cells. Finally, an inhibitor of microfilament polymerization abrogated proliferation to CnM. These results indicate that the mitogenic activity of C. neoformans requires phagocytosis of the organism, lysosomal or endosomal processing, proteolytic activity, and microfilament polymerization and intracellular transport as a prerequisite for T-cell proliferation.

Phagosomes acquire nascent and recycling class II MHC molecules but primarily use nascent molecules in phagocytic antigen processing

Phagosomes contain class II MHC (MHC-II) and form peptide:MHC-II complexes, but the source of phagosomal MHC-II molecules is uncertain. Phagosomes may acquire nascent MHC-II or preexisting, recycling MHC-II that may be internalized from the plasma membrane. Brefeldin A (BFA) was used to deplete nascent MHC-II in murine macrophages to determine the relative contributions of nascent and recycling MHC-II molecules to phagocytic Ag processing. In addition, biotinylation of cell-surface proteins was used to assess the transport of MHC-II from the cell surface to phagosomes. BFA inhibited macrophage processing of latex bead-conjugated Ag for presentation to T cells, suggesting that nascent MHC-II molecules are important in phagocytic Ag processing. Furthermore, detection of specific peptide:MHC-II complexes in isolated phagosomes confirmed that BFA decreased formation of peptide:MHC-II complexes within phagosomes. Both flow organellometry and Western blot analysis of purified phagosomes showed that about two-thirds of phagosomal MHC-II was nascent (depleted by 3 h prior treatment with BFA) and primarily derived from intracellular sites. About one-third of phagosomal MHC-II was preexisting and primarily derived from the plasma membrane. BFA had little effect on phagosomal H2-DM or the degradation of bead-associated Ag. Thus, inhibition of phagocytic Ag processing by BFA correlated with depletion of nascent MHC-II in phagosomes and occurred despite the persistent delivery of plasma membrane-derived recycling MHC-II molecules and other Ag-processing components to phagosomes. These observations suggest that phagosomal Ag processing depends primarily on nascent MHC-II molecules delivered from intracellular sites, e.g., endocytic compartments.

Phagocytic processing of antigens for presentation by class II major histocompatibility complex molecules

Microbes and other particulate antigens (Ags) are internalized by phagocytosis and then reside in plasma membrane-derived phagosomes. The contribution of phagosomes to the degradation of Ags has long been appreciated. It has been unclear, however, whether peptides derived from these degraded antigens bind class II major histocompatibility complex (MHC-II) molecules within phagosomes or within endocytic compartments that receive Ag fragments from phagosomes. Recent experiments have demonstrated that phagosomes containing Ag-conjugated latex beads express a full complement of Ag-processing molecules, e.g. MHC-II molecules, invariant chain, H2-DM and proteases sufficient to degrade bead- associated Ag. These phagosomes mediate the formation of peptide-MHC-II complexes, which are transported to the cell surface and presented to T cells. Phagosomes acquire both newly synthesized and plasma membrane-derived MHC-II molecules, but the formation of peptide-MHC-II complexes in phagosomes primarily involves newly synthesized MHC-II molecules. The content and traffic of phagosomal proteins vary considerably with the type of Ag ingested. Pathogenic microbes can alter phagosome composition and function to reduce Ag processing. For example, Mycobacterium tuberculosis blocks the maturation of phagosomes and reduces the ability of infected cells to present exogenous soluble protein Ags.

The relative abundance of two measles virus fusion protein peptide-DR1 complexes expressed by B cells is independent of the form of the antigen

The relative processing and presentation efficiency of two DR1-restricted determinants from the fusion protein (F protein) of measles virus (MV) was determined using three forms of antigen (Ag): MV, an F protein recombinant vaccinia virus, and a chimerical polypeptide between the glutathione S-transferase and the F protein (GST-F protein). First, it was shown that these different preparations of F protein have distinct processing requirements. In MV-infected B cells, the F254 determinant (contained within the F protein sequence 254-268) relies on protein synthesis for its presentation, while the F314 determinant (contained within the F protein sequence 314-328) is also presented in the absence of protein synthesis. By contrast, in GST-F protein-pulsed B cells, presentation of both determinants is dependent on protein synthesis. Then, it was established that, independently of the form of the Ag, the F314 determinant was considerably more (18- to 36-fold) efficiently processed and presented than the F254 determinant. These results indicate that determinants from the same protein are displayed by antigen-presenting cells at widely different levels and they may also suggest that this is an intrinsic characteristic of the determinants, rather than a feature controlled by the processing pathways followed by the Ag.

Phagocytic antigen processing and effects of microbial products on antigen processing and T-cell responses

Processing of exogenous antigens and microbes involves contributions by multiple different endocytic and phagocytic compartments. During the processing of soluble antigens, different endocytic compartments have been demonstrated to use distinct antigen-processing mechanisms and to process distinct sets of antigenic epitopes. Processing of particulate and microbial antigens involves phagocytosis and functions contributed by phagocytic compartments. Recent data from our laboratory demonstrate that phagosomes containing antigen-conjugated latex beads are fully competent class II MHC (MHC-II) antigen-processing organelles, which generate peptide:MHC-II complexes. In addition, phagocytosed antigen enters an alternate class I MHC (MHC-I) processing pathway that results in loading of peptides derived from exogenous antigens onto MHC-I molecules, in contrast to the cytosolic antigen source utilized by the conventional MHC-I antigen-processing pathway. Antigen processing and other immune response mechanisms may be activated or inhibited by microbial components to the benefit of either the host or the pathogen. For example, antigen processing and T-cell responses (e.g. Th1 vs Th2 differentiation) are modulated by multiple distinct microbial components, including lipopolysaccharide, cholera toxin, heat labile enterotoxin of Escherichia coli, DNA containing CpG motifs (found in prokaryotic and invertebrate DNA but not mammalian DNA) and components of Mycobacterium tuberculosis.

Processing of the DRB1*1103-restricted measles virus nucleoprotein determinant 185-199 in the endosomal compartment

MHC class II molecules present to CD4+ T cells protein fragments which mostly derive from the extracellular and from the endosomal compartments. Determinants of cytosolic proteins are, however, also displayed by MHC class II molecules following pathways which are still not yet fully characterized. Here we describe the isolation of DRB1*1103-restricted T cell clones specific for the measles virus (MV) nucleoprotein peptide 185-199 (N185). Experiments were then conducted to delineate how this determinant is assembled with DR molecules. In vitro binding analyses indicated that complexes between the N185 peptide and DRB1*1103 protein are optimally constituted at pH 4-4.5. In cellular experiments it was observed that chloroquine, leupeptin and emetine, which are classical inhibitors of presentation of MHC class II-restricted antigens, when added during infection of B cells with MV, prevent presentation of the N185 determinant. In addition, it was found that the N185 determinant is efficiently presented when the nucleoprotein is exogenously provided to B cells, either by blocking MV fusion with the peptide FFG or by the use of purified nucleoprotein. In contrast, it was observed that nucleoprotein recombinant vaccinia virus (vv-N)-infected B cells weakly stimulated N185-specific T cells, indicating that the restricted localization of the nucleoprotein in the cytosol resulted in a poor presentation of the N185 determinant. Taken together, these findings suggest that it is prior to delivery of the nucleoprotein into the cytosol that the N185 determinant is efficiently assembled with newly synthesized DR molecules in the acidic environment of the endosomal compartment.

The majority of HLA-DR3 alloreactive T cells is peptide specific, but does not recognize known DR3-bound sequences

Rejection of transplants is frequently caused by activation of alloreactive T cells that recognize HLA/peptide differences between patient and graft. This T-cell response can be directed towards the HLA molecule, the HLA-bound peptide or towards a combination. More insight in the involvement of peptides in this process may help to find ways to avoid rejection using for example antagonist peptides. In recent years many naturally processed HLA-bound peptides have been identified. This raises the question of whether these, presumably abundant, peptides are involved in class II-specific allorecognition. To investigate this, we first determined the proportion of peptide-specific alloreactive T cells in the alloresponse against HLA-DR3. For this purpose we have tested a panel of DR3-specific alloreactive T-cell clones against a DM-mutant (i.e. peptide loading deficient) cell line. We found that 59 out of 64 alloreactive T-cell clones were dependent upon the presence of DM for an optimal response. However, only 2 DM-dependent T-cell clones recognize known peptide sequences. Thus we conclude that most DR3-specific alloreactive T-cell clones are peptide specific and that the currently known DR3-bound peptides are not the main target for allorecognition. Finally, we identified 4 T-cell clones that recognized the DM-mutant better than the wild-type cell line. The response against the wild-type cell line could not be restored with invariant chain derived peptides (CLIP). This provides additional evidence that DM can negatively select self-peptides other than CLIP, which can result in selection against peptides involved in allorecognition.

Peptide-MHC complexes assembled following multiple pathways: an opportunity for the design of vaccines and therapeutic molecules

Antigen degradation and peptide loading to major histocompatibility complex class I and class II molecules are described with special emphasis on "noncanonical" pathways. Examples of specific peptide loading for measles proteins are provided. In addition, characterization of defined epitopes presented to T cells can lead to the design of products of special interest in medicine and, in particular, in development of vaccines.