Defective major histocompatibility complex class II assembly, transport, peptide acquisition, and CD4+ T cell selection in mice lacking invariant chain expression

Infection and inflammation stimulate expansion of a CD74+ Paneth cell subset to regulate disease progression

Paneth cells (PCs), a specialized secretory cell type in the small intestine, are increasingly recognized as having an essential role in host responses to microbiome and environmental stresses. Whether and how commensal and pathogenic microbes modify PC composition to modulate inflammation remain unclear. Using newly developed PC-reporter mice under conventional and gnotobiotic conditions, we determined PC transcriptomic heterogeneity in response to commensal and invasive microbes at single cell level. Infection expands the pool of CD74+ PCs, whose number correlates with auto or allogeneic inflammatory disease progressions in mice. Similar correlation was found in human inflammatory disease tissues. Infection-stimulated cytokines increase production of reactive oxygen species (ROS) and expression of a PC-specific mucosal pentraxin (Mptx2) in activated PCs. A PC-specific ablation of MyD88 reduced CD74+ PC population, thus ameliorating pathogen-induced systemic disease. A similar phenotype was also observed in mice lacking Mptx2. Thus, infection stimulates expansion of a PC subset that influences disease progression.

Molecular Determinants Regulating the Plasticity of the MHC Class II Immunopeptidome

In the last few years, advancement in the analysis of the MHC class II (MHC-II) ligandome in several mouse and human haplotypes has increased our understanding of the molecular components that regulate the range and selection of the MHC-II presented peptides, from MHC class II molecule polymorphisms to the recognition of different conformers, functional differences in endosomal processing along the endocytic tract, and the interplay between the MHC class II chaperones DM and DO. The sum of all these variables contributes, qualitatively and quantitatively, to the composition of the MHC II ligandome, altogether ensuring that the immunopeptidome landscape is highly sensitive to any changes in the composition of the intra- and extracellular proteome for a comprehensive survey of the microenvironment for MHC II presentation to CD4 T cells.

Commensal oral microbiota induces osteoimmunomodulatory effects separate from systemic microbiome in mice

Commensal microbes critically regulate skeletal homeostasis, yet the impact of specific microbiota communities on osteoimmune response mechanisms is unknown. To discern osteoimmunomodulatory effects imparted by the commensal oral microbiota that are distinct from the systemic microbiota, osteoimmunology studies were performed in both alveolar bone and nonoral skeletal sites of specific pathogen–free (SPF) versus germ-free (GF) mice and SPF mice subjected to saline versus chlorhexidine oral rinses. SPF versus GF mice had reduced cortical/trabecular bone and an enhanced pro-osteoclastic phenotype in alveolar bone. TLR signaling and Th17 cells that have known pro-osteoclastic actions were increased in alveolar BM, but not long BM, of SPF versus GF mice. MHC II antigen presentation genes and activated DCs and CD4⁺ T cells were elevated in alveolar BM, but not long BM, of SPF versus GF mice. These findings were substantiated by in vitro allostimulation studies demonstrating increased activated DCs derived from alveolar BM, but not long BM, of SPF versus GF mice. Chlorhexidine antiseptic rinse depleted the oral, but not gut, bacteriome in SPF mice. Findings from saline- versus chlorhexidine-treated SPF mice corroborated outcomes from SPF versus GF mice, which reveals that the commensal oral microbiota imparts osteoimmunomodulatory effects separate from the systemic microbiome.

GILT Expression in Human Melanoma Cells Enhances Generation of Antigenic Peptides for HLA Class II-Mediated Immune Recognition

Melanoma is an aggressive skin cancer that has become increasingly prevalent in western populations. Current treatments such as surgery, chemotherapy, and high-dose radiation have had limited success, often failing to treat late stage, metastatic melanoma. Alternative strategies such as immunotherapies have been successful in treating a small percentage of patients with metastatic disease, although these treatments to date have not been proven to enhance overall survival. Several melanoma antigens (Ags) proposed as targets for immunotherapeutics include tyrosinase, NY-ESO-1, gp-100, and Mart-1, all of which contain both human leukocyte antigen (HLA) class I and class II-restricted epitopes necessary for immune recognition. We have previously shown that an enzyme, gamma-IFN-inducible lysosomal thiol-reductase (GILT), is abundantly expressed in professional Ag presenting cells (APCs), but absent or expressed at greatly reduced levels in many human melanomas. In the current study, we report that increased GILT expression generates a greater pool of antigenic peptides in melanoma cells for enhanced CD4+ T cell recognition. Our results suggest that the induction of GILT in human melanoma cells could aid in the development of a novel whole-cell vaccine for the enhancement of immune recognition of metastatic melanoma.

The transmembrane domain and luminal C-terminal region independently support invariant chain trimerization and assembly with MHCII into nonamers

Background

Invariant chain (CD74, Ii) is a multifunctional protein expressed in antigen presenting cells. It assists the ER exit of various cargos and serves as a receptor for the macrophage migration inhibitory factor. The newly translated Ii chains trimerize, a structural feature that is not readily understood in the context of its MHCII chaperoning function. Two segments of Ii, the luminal C-terminal region (TRIM) and the transmembrane domain (TM), have been shown to participate in the trimerization process but their relative importance and impact on the assembly with MHCII molecules remains debated. Here, we addressed the requirement of these domains in the trimerization of human Ii as well as in the oligomerization with MHCII molecules. We used site-directed mutagenesis to generate series of Ii and DR mutants. These were transiently transfected in HEK293T cells to test their cell surface expression and analyse their interactions by co-immunoprecipitations.

Results

Our results showed that the TRIM domain is not essential for Ii trimerization nor for intracellular trafficking with MHCII molecules. We also gathered evidence that in the absence of TM, TRIM allows the formation of multi-subunit complexes with HLA-DR. Similarly, in the absence of TRIM, Ii can assemble into high-order structures with MHCII molecules.

Conclusions

Altogether, our data show that trimerization of Ii through either TM or TRIM sustains nonameric complex formation with MHCII molecules.

Supplementary Information

The online version contains supplementary material available at 10.1186/s12865-021-00444-6.

CD74 regulates cellularity and maturation of medullary thymic epithelial cells partially by activating the canonical NF-κB signaling pathway

Thymic epithelial cells (TECs) are indispensable for T cell development, T cell receptor (TCR) repertoire selection, and specific lineage differentiation. Medullary thymic epithelial cells (mTECs), which account for the majority of TECs in adults, are critical for thymocyte selection and self-tolerance. CD74 is a nonpolymorphic transmembrane glycoprotein of major histocompatibility complex class II (MHCII) that is expressed in TECs. However, the exact role of CD74 in regulating the development of mTEC is poorly defined. In this research, we found that loss of CD74 resulted in a significant diminution in the medulla, a selective reduction in the cell number of mature mTECs expressing CD80 molecules, which eventually led to impaired thymic CD4⁺ T cell development. Moreover, RNA-sequence analysis showed that CD74 deficiency obviously downregulated the canonical nuclear factor kappa-light-chain-enhancer of activated B cells (NF-κB) signaling pathway in mTECs. Our results suggest that CD74 positively controls mTEC cellularity and maturation partially by activating the canonical NF-κB signaling pathway.

Type II alveolar cell MHCII improves respiratory viral disease outcomes while exhibiting limited antigen presentation

Type II alveolar cells (AT2s) are critical for basic respiratory homeostasis and tissue repair after lung injury. Prior studies indicate that AT2s also express major histocompatibility complex class II (MHCII) molecules, but how MHCII expression by AT2s is regulated and how it contributes to host defense remain unclear. Here we show that AT2s express high levels of MHCII independent of conventional inflammatory stimuli, and that selective loss of MHCII from AT2s in mice results in modest worsening of respiratory virus disease following influenza and Sendai virus infections. We also find that AT2s exhibit MHCII presentation capacity that is substantially limited compared to professional antigen presenting cells. The combination of constitutive MHCII expression and restrained antigen presentation may position AT2s to contribute to lung adaptive immune responses in a measured fashion, without over-amplifying damaging inflammation.

Deficiency of the Intramembrane Protease SPPL2a Alters Antimycobacterial Cytokine Responses of Dendritic Cells

Signal peptide peptidase-like 2a (SPPL2a) is an aspartyl intramembrane protease essential for degradation of the invariant chain CD74. In humans, absence of SPPL2a leads to Mendelian susceptibility to mycobacterial disease, which is attributed to a loss of the dendritic cell (DC) subset conventional DC2. In this study, we confirm depletion of conventional DC2 in lymphatic tissues of SPPL2a^-/- mice and demonstrate dependence on CD74 using SPPL2a^-/- CD74^-/- mice. Upon contact with mycobacteria, SPPL2a^-/- bone marrow-derived DCs show enhanced secretion of IL-1β, whereas production of IL-10 and IFN-β is reduced. These effects correlated with modulated responses upon selective stimulation of the pattern recognition receptors TLR4 and Dectin-1. In SPPL2a^-/- bone marrow-derived DCs, Dectin-1 is redistributed to endosomal compartments. Thus, SPPL2a deficiency alters pattern recognition receptor pathways in a CD74-dependent way, shifting the balance from anti- to proinflammatory cytokines in antimycobacterial responses. We propose that in addition to the DC reduction, this altered DC functionality contributes to Mendelian susceptibility to mycobacterial disease upon SPPL2a deficiency.

B. burgdorferi sensu lato-induced inhibition of antigen presentation is mediated by RIP1 signaling resulting in impaired functional T cell responses towards Candida albicans

Antigen presentation is a crucial innate immune cell function that instructs adaptive immune cells. Loss of this pathway severely impairs the development of adaptive immune responses. To investigate whether B. burgdorferi sensu lato. spirochetes modulate the induction of an effective immune response, primary human PBMCs were isolated from healthy volunteers and stimulated with B. burgdorferi s.l. Through cell entry, TNF receptor I, and RIP1 signaling cascades, B. burgdorferi s.l. strongly downregulated genes and proteins involved in antigen presentation, specifically HLA-DM, MHC class II and CD74. Antigen presentation proteins were distinctively inhibited in monocyte subsets, monocyte-derived macrophages, and dendritic cells. When compared to a range of other pathogens, B. burgdorferi s.l.-induced suppression of antigen presentation appears to be specific. Inhibition of antigen presentation interfered with T-cell recognition of B. burgdorferi s.l., and memory T-cell responses against Candidaalbicans. Re-stimulation of PBMCs with the commensal microbe C.albicans following B. burgdorferi s.l. exposure resulted in significantly reduced IFN-γ, IL-17 and IL-22 production. These findings may explain why patients with Lyme borreliosis develop delayed adaptive immune responses. Unravelling the mechanism of B. burgdorferi s.l.-induced inhibition of antigen presentation, via cell entry, TNF receptor I, and RIP1 signaling cascades, explains the difficulty to diagnose the disease based on serology and to obtain an effective vaccine against Lyme borreliosis.

Unveiling the Hidden Treasury: CIITA-Driven MHC Class II Expression in Tumor Cells to Dig up the Relevant Repertoire of Tumor Antigens for Optimal Stimulation of Tumor Specific CD4+ T Helper Cells

Despite the recent enthusiasm generated by novel immunotherapeutic approaches against cancer based on immune checkpoint inhibitors, it becomes increasingly clear that single immune-based strategies are not sufficient to defeat the various forms and types of tumors. Within this frame, novel vaccination strategies that are based on optimal stimulation of the key cell governing adaptive immunity, the CD4+ T helper cell, will certainly help in constructing more efficient treatments. In this review, we will focus on this aspect, mainly describing our past and recent contributions that, starting with a rather unorthodox approach, have ended up with the proposition of a new idea for making available an unprecedented extended repertoire of tumor antigens, both in quantitative and qualitative terms, to tumor-specific CD4+ T helper cells. Our approach is based on rendering the very same tumor cells antigen presenting cells for their own tumor antigens by gene transfer of CIITA, the major transcriptional coordinator of MHC class II expression discovered in our laboratory. CIITA-driven MHC class II-expressing tumor cells optimally stimulate in vivo tumor specific MHC class II-restricted CD4 T cells generating specific and long lasting protective immunity against the tumor. We will discuss the mechanism underlying protection and elaborate not only on the applicability of this approach for novel vaccination strategies amenable to clinical setting, but also on the consequence of our discoveries on sedimented immunological dogmas that are related to antigen presentation.

Invariant chain regulates endosomal fusion and maturation through an interaction with the SNARE Vti1b

The invariant chain (Ii, also known as CD74) is a multifunctional regulator of adaptive immune responses and is responsible for sorting major histocompatibility complex class I and class II (MHCI and MHCII, respectively) molecules, as well as other Ii-associated molecules, to a specific endosomal pathway. When Ii is expressed, endosomal maturation and proteolytic degradation of proteins are delayed and, in non-antigen presenting cells, the endosomal size increases, but the molecular mechanisms underlying this are not known. We identified that a SNARE, Vti1b, is essential for regulating these Ii-induced effects. Vti1b binds to Ii and is localized at the contact sites of fusing Ii-positive endosomes. Furthermore, truncated Ii lacking the cytoplasmic tail, which is not internalized from the plasma membrane, relocates Vti1b to the plasma membrane. Knockout of Ii in an antigen-presenting cell line was found to speed up endosomal maturation, whereas silencing of Vti1b inhibits the Ii-induced maturation delay. Our results suggest that Ii, by interacting with the SNARE Vti1b in antigen-presenting cells, directs specific Ii-associated SNARE-mediated fusion in the early part of the endosomal pathway that leads to a slower endosomal maturation for efficient antigen processing and MHC antigen loading.

MHC class II transactivator CIITA induces cell resistance to Ebola virus and SARS-like coronaviruses

Recent outbreaks of Ebola virus (EBOV) and severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) have exposed our limited therapeutic options for such diseases and our poor understanding of the cellular mechanisms that block viral infections. Using a transposon-mediated gene-activation screen in human cells, we identify that the major histocompatibility complex (MHC) class II transactivator (CIITA) has antiviral activity against EBOV. CIITA induces resistance by activating expression of the p41 isoform of invariant chain CD74, which inhibits viral entry by blocking cathepsin-mediated processing of the Ebola glycoprotein. We further show that CD74 p41 can block the endosomal entry pathway of coronaviruses, including SARS-CoV-2. These data therefore implicate CIITA and CD74 in host defense against a range of viruses, and they identify an additional function of these proteins beyond their canonical roles in antigen presentation.

CD74 is dispensable for development of chronic lymphocytic leukemia in Eµ-TCL1 transgenic mice

CD74 is a surface protein expressed on immune cells, which acts as receptor for the chemokine macrophage migration inhibitory factor (MIF). Signaling via the MIF/CD74-axis has been reported to be important for the pathogenesis of chronic lymphocytic leukemia (CLL). We wanted to clarify the role of CD74 in MIF-induced signaling/leukemic development. In Eμ-TCL1 transgenic mice, occurrence of the leukemic phenotype was associated with increased surface CD74 expression. Eμ-TCL1^+/+Cd74^-/- mice showed similar kinetics and clinical features of CLL development as Eμ-TCL1^+/+ mice. MIF stimulation of leukemic splenocytes led to AKT activation in a CD74-dependent manner. AKT activation was reduced in Cd74-deficient splenocytes in the presence of the oncogenic TCL1-transgene. Tumor cell apoptosis/proliferation were unaffected in Eμ-TCL1^+/+Cd74^-/- mice. Our data suggest that the need for active CD74 signaling is overcome in the leukemic context of TCL1-driven CLL, and that CD74 may have a dispensable role for CLL pathogenesis in this model.

Bacterial lipopolysaccharide induces the intracellular expression of trophoblastic specific CD74 isoform in human first trimester trophoblast cells: Correlation with unsuccessful early pregnancy

OBJECTIVE

During first trimester of human pregnancy, the maternal system develops immunity against infection and to provide protection of allogeneic foetus from abortion. This study was undertaken to determine the role of trophoblast specific CD74 isoforms in first trimester trophoblast derived cells under normal and lipopolysaccharide (LPS) stimulated conditions.

METHODS

Gene and protein of CD74 were determined in first trimester trophoblast derived cells, JEG-3 and ACH-3 P and also in human placenta by PCR, western blotting and immunoprecipitation. Effect of LPS mediated infection on the regulation of CD74 isoforms was studied intracellularly and also on the cells surface by flow cytometry.

RESULTS

Data demonstrated that JEG-3 and ACH-3 P cells under normal conditions have not expressed CD74 isoforms neither intracellularly or nor on the surface. These results were further validated directly in human placenta. However, treatment of these trophoblast cells with a bacterial LPS, significantly upregulated CD74 mRNA expression (p < 0.05). Furthermore, expression of CD74 on the surface was not detected even after stimulation with LPS. Interestingly, CD74 isoform at 35 kDa was significantly detected intracellularly upon stimulation with LPS (p < 0.05). These results were further confirmed by western blotting followed by immunoprecipitation.

CONCLUSIONS

To the best of our knowledge, this is the first study concluded that the bacterial LPS induce infection in the first trimester trophoblasts via intracellular upregulation of CD74. Data indicated that the lack of cell surface expression of trophoblastic specific isoforms of CD74 may provide protection for human pregnancy in the first trimester.

HLA-DM catalytically enhances peptide dissociation by sensing peptide-MHC class II interactions throughout the peptide-binding cleft

Human leukocyte antigen-DM (HLA-DM) is an integral component of the major histocompatibility complex class II (MHCII) antigen-processing and -presentation pathway. HLA-DM shapes the immune system by differentially catalyzing peptide exchange on MHCII molecules, thereby editing the peptide-MHCII (pMHCII) repertoire by imposing a bias on the foreign and self-derived peptide cargos that are presented on the cell surface for immune surveillance and tolerance induction by CD4⁺ T cells. To better understand DM selectivity, here we developed a real-time fluorescence anisotropy assay to delineate the pMHCII intrinsic stability, DM-binding affinity, and catalytic turnover, independent kinetic parameters of HLA-DM enzymatic activity. We analyzed prominent pMHCII contacts by differentiating the kinetic parameters in pMHCII homologs, observing that peptide interactions throughout the MHCII-binding cleft influence both the rate of peptide dissociation from the DM-pMHCII catalytic complex and the binding affinity of HLA-DM for a pMHCII. We show that the intrinsic stability of a pMHCII linearly correlates with DM catalytic turnover, but is nonlinearly correlated with its binding affinity. Surprisingly, interactions at the peptides N terminus up to and including MHCII position one (P1) anchor affected the catalytic turnover, suggesting that the active DM-pMHCII catalytic complex operates on pMHCII complexes with full peptide occupancy. Furthermore, interactions at the peptide C terminus modulated DM-binding affinity, suggesting distal communication between peptide interactions with the MHCII and the DM-pMHCII binding interface. Our results imply an intimate linkage between the DM-pMHCII interface and peptide-MHCII interactions throughout the peptide-binding cleft.

Downregulation of MHC Class II by Ubiquitination Is Required for the Migration of CD206+ Dendritic Cells to Skin-Draining Lymph Nodes

Dendritic cells (DCs) are critical players in skin homeostasis. A subset of mannose receptor (CD206)-expressing monocyte-derived DCs was found in skin, and their migratory counterpart is present in skin-draining lymph nodes (sdLNs). Skin CD206⁺ DCs were shown to upregulate MHC class II (MHCII) progressively, raising the question of whether this feature affects their biology. In this study, we assessed the role of MHCII regulation in the development and migration of these cells in mouse models expressing differential MHCII levels. Using CD206 as a surrogate marker, we found that skin CD206⁺ DCs develop in an MHCII-independent manner. However, their migration to sdLNs was affected by overexpression rather than absence or lower expression of MHCII. Accordingly, B16 tumor growth was exacerbated in mice overexpressing MHCII in the absence of ubiquitination. Mechanistically, CD206⁺ DCs from these mice showed decreased IRF4 and CCR7 expression. LPS, which is known to promote monocyte-derived DC recruitment to sdLNs, partially improved these defects. However, GM-CSF delivery restored CD206⁺ DC migration by promoting IRF4 expression. Collectively, these data show that MHCII downregulation is crucial for IRF4-dependent migration of CD206⁺ DCs to sdLNs in health and disease.

CIITA-Driven MHC Class II Expressing Tumor Cells as Antigen Presenting Cell Performers: Toward the Construction of an Optimal Anti-tumor Vaccine

Construction of an optimal vaccine against tumors relies on the availability of appropriate tumor-specific antigens capable to stimulate CD4+ T helper cells (TH) and CD8+ cytolytic T cells (CTL). CTL are considered the major effectors of the anti-tumor adaptive immune response as they recognize antigens presented on MHC class I (MHC-I) molecules usually expressed in all cells and thus also in tumors. However, attempts to translate in clinics vaccination protocols based only on tumor-specific MHC-I-bound peptides have resulted in very limited, if any, success. We believe failure was mostly due to inadequate triggering of the TH arm of adaptive immunity, as TH cells are necessary to trigger and maintain the proliferation of all the immune effector cells required to eliminate tumor cells. In this review, we focus on a novel strategy of anti-tumor vaccination established in our laboratory and based on the persistent expression of MHC class II (MHC-II) molecules in tumor cells. MHC-II are the restricting elements of TH recognition. They are usually not expressed in solid tumors. By genetically modifying tumor cells of distinct histological origin with the MHC-II transactivator CIITA, the physiological controller of MHC-II gene expression discovered in our laboratory, stable expression of all MHC class II genes was obtained. This resulted in tumor rejection or strong retardation of tumor growth in vivo in mice, mediated primarily by tumor-specific TH cells as assessed by both depletion and adoptive cell transfer experiments. Importantly these findings led us to apply this methodology to human settings for the purification of MHC-II-bound tumor specific peptides directly from tumor cells, specifically from hepatocarcinomas, and the construction of a multi-peptide (MHC-II and MHC-I specific) immunotherapeutic vaccine. Additionally, our approach unveiled a noticeable exception to the dogma that dendritic cells are the sole professional antigen presenting cells (APC) capable to prime naïve TH cells, because CIITA-dependent MHC-II expressing tumor cells could also perform this function. Thus, our approach has served not only to select the most appropriate tumor specific peptides to activate the key lymphocytes triggering the anti-tumor effector functions but also to increase our knowledge of intimate mechanisms governing basic immunological processes.

In Vitro Digestion with Proteases Producing MHC Class II Ligands

Proteases generate peptides that bind to MHC class II molecules to interact with a wide diversity of CD4⁺ T cells. They are expressed in dedicated organelles: endosomes and lysosomes of professional antigen-presenting cells (pAPCs) such as B cells, macrophages, and dendritic cells. The identification of endosomal proteases which produce antigenic peptides is important for example for better vaccination and to prevent autoimmune diseases. Here, we describe a panel of techniques (in vitro digestion assays of protein with recombinant proteases or purified endosomes/lysosomes, T cell stimulation) to monitor the production of MHC class II ligands.

Biological Consequences of MHC-II Expression by Tumor Cells in Cancer

Immunotherapy has emerged as a key pillar of cancer treatment. To build upon the recent successes of immunotherapy, intense research efforts are aimed at a molecular understanding of antitumor immune responses, identification of biomarkers of immunotherapy response and resistance, and novel strategies to circumvent resistance. These studies are revealing new insight into the intricacies of tumor cell recognition by the immune system, in large part through MHCs. Although tumor cells widely express MHC-I, a subset of tumors originating from a variety of tissues also express MHC-II, an antigen-presenting complex traditionally associated with professional antigen-presenting cells. MHC-II is critical for antigen presentation to CD4+ T lymphocytes, whose role in antitumor immunity is becoming increasingly appreciated. Accumulating evidence demonstrates that tumor-specific MHC-II associates with favorable outcomes in patients with cancer, including those treated with immunotherapies, and with tumor rejection in murine models. Herein, we will review current research regarding tumor-enriched MHC-II expression and regulation in a range of human tumors and murine models, and the possible therapeutic applications of tumor-specific MHC-II.

Antibiotic Perturbation of Gut Microbiota Dysregulates Osteoimmune Cross Talk in Postpubertal Skeletal Development

Commensal gut microbiota-host immune responses are experimentally delineated via gnotobiotic animal models or alternatively by antibiotic perturbation of gut microbiota. Osteoimmunology investigations in germ-free mice, revealing that gut microbiota immunomodulatory actions critically regulate physiologic skeletal development, highlight that antibiotic perturbation of gut microbiota may dysregulate normal osteoimmunological processes. We investigated the impact of antibiotic disruption of gut microbiota on osteoimmune response effects in postpubertal skeletal development. Sex-matched C57BL/6T mice were administered broad-spectrum antibiotics or vehicle-control from the age of 6 to 12 weeks. Antibiotic alterations in gut bacterial composition and skeletal morphology were sex dependent. Antibiotics did not influence osteoblastogenesis or endochondral bone formation, but notably enhanced osteoclastogenesis. Unchanged Tnf or Ccl3 expression in marrow and elevated tumor necrosis factor-α and chemokine (C-C motif) ligand 3 in serum indicated that the pro-osteoclastic effects of the antibiotics are driven by increased systemic inflammation. Antibiotic-induced broad changes in adaptive and innate immune cells in mesenteric lymph nodes and spleen demonstrated that the perturbation of gut microbiota drives a state of dysbiotic hyperimmune response at secondary lymphoid tissues draining local gut and systemic circulation. Antibiotics up-regulated the myeloid-derived suppressor cells, immature myeloid progenitor cells known for immunosuppressive properties in pathophysiologic inflammatory conditions. Myeloid-derived suppressor cell-mediated immunosuppression can be antigen specific. Therefore, antibiotic-induced broad suppression of major histocompatibility complex class II antigen presentation genes in bone marrow discerns that antibiotic perturbation of gut microbiota dysregulates critical osteoimmune cross talk.

Mechanisms underlying the lack of endogenous processing and CLIP-mediated binding of the invariant chain by HLA-DP84Gly

While the principles of classical antigen presentation via MHC class II are well-established, the mechanisms for the many routes of cross-presentation by which endogenous antigens become associated with class II molecules are not fully understood. We have recently demonstrated that the single amino acid polymorphism HLA-DPβ^84Gly (DP^84Gly) is critical to abrogate class II invariant chain associated peptide (CLIP) region-mediated binding of invariant chain (Ii) to DP, allowing endoplasmic reticulum (ER)-resident endogenous antigens to constitutively associate with DP^84Gly such as DP4. In this study, we demonstrate that both the CLIP and N-terminal non-CLIP Ii regions cooperatively generate an Ii conformation that cannot associate with DP^84Gly via the CLIP region. We also demonstrate the ability of DP4 to efficiently process and present antigens encoded in place of CLIP in a chimeric Ii, regardless of wild type Ii and HLA-DM expression. These data highlight the complex interplay between DP polymorphisms and the multiple Ii regions that cooperatively regulate this association, ultimately controlling the presentation of endogenous antigens on DP molecules. These results may also offer a mechanistic explanation for recent studies identifying the differential effects between DP^84Gly and DP^84Asp as clinically relevant in human disease.

Cutting Edge: Allograft Rejection Is Associated with Weak T Cell Responses to Many Different Graft Leukocyte-Derived Peptides

Organ transplants are rapidly rejected because T cells in the recipient attack the foreign MHC molecules on the graft. The robustness of the T cell response to histoincompatible tissue is not understood. We found that mice have many small T cell populations with Ag receptors specific for a foreign MHC class II molecule type loaded with peptides from leukocytes from the graft. These T cells proliferated modestly after skin transplantation and underwent relatively weak functional differentiation compared with T cells stimulated by a vaccine. Thus, the potency of the T cell response to histoincompatible tissue is likely due to many small T cell populations responding weakly to hundreds of MHC-bound peptides from graft-derived leukocytes.

Tumor Immunology meets…Immunology: Modified cancer cells as professional APC for priming naïve tumor-specific CD4+ T cells

Although recent therapeutic approaches have revitalized the enthusiasm of the immunological way to combat cancer, still the comprehension of immunity against tumors is largely incomplete. Due to their specific function, CD8+ T cells with cytolytic activity (CTL) have attracted the attention of most investigators because CTL are considered the main effectors against tumor cells. Nevertheless, CTL activity and persistence is largely dependent on the action of CD4+ T helper cells (TH). Thus establishment of tumor-specific TH cell response is key to the optimal response against cancer. Here we describe emerging new strategies to increase the TH cell recognition of tumor antigens. In particular, we review recent data indicating that tumor cells themselves can act as surrogate antigen presenting cells for triggering TH response and how these findings can help in constructing immunotherapeutic protocols for anti-cancer vaccine development.

Mechanistic understanding and significance of small peptides interaction with MHC class II molecules for therapeutic applications

Major histocompatibility complex (MHC) class II molecules are expressed by antigen-presenting cells and stimulate CD4(+) T cells, which initiate humoral immune responses. Over the past decade, interest has developed to therapeutically impact the peptides to be exposed to CD4(+) T cells. Structurally diverse small molecules have been discovered that act on the endogenous peptide exchanger HLA-DM by different mechanisms. Exogenously delivered peptides are highly susceptible to proteolytic cleavage in vivo; however, it is only when successfully incorporated into stable MHC II-peptide complexes that these peptides can induce an immune response. Many of the small molecules so far discovered have highlighted the molecular interactions mediating the formation of MHC II-peptide complexes. As potential drugs, these small molecules open new therapeutic approaches to modulate MHC II antigen presentation pathways and influence the quality and specificity of immune responses. This review briefly introduces how CD4(+) T cells recognize antigen when displayed by MHC class II molecules, as well as MHC class II-peptide-loading pathways, structural basis of peptide binding and stabilization of the peptide-MHC complexes. We discuss the concept of MHC-loading enhancers, how they could modulate immune responses and how these molecules have been identified. Finally, we suggest mechanisms whereby MHC-loading enhancers could act upon MHC class II molecules.

CD74-Downregulation of Placental Macrophage-Trophoblastic Interactions in Preeclampsia

RATIONALE

We hypothesized that cluster of differentiation 74 (CD74) downregulation on placental macrophages, leading to altered macrophage-trophoblast interaction, is involved in preeclampsia.

OBJECTIVE

Preeclamptic pregnancies feature hypertension, proteinuria, and placental anomalies. Feto-placental macrophages regulate villous trophoblast differentiation during placental development. Disturbance of this well-balanced regulation can lead to pathological pregnancies.

METHODS AND RESULTS

We performed whole-genome expression analysis of placental tissue. CD74 was one of the most downregulated genes in placentas from preeclamptic women. By reverse transcriptase-polymerase chain reaction, we confirmed this finding in early-onset (<34 gestational week, n=26) and late-onset (≥34 gestational week, n=24) samples from preeclamptic women, compared with healthy pregnant controls (n=28). CD74 protein levels were analyzed by Western blot and flow cytometry. We identified placental macrophages to express CD74 by immunofluorescence, flow cytometry, and RT-PCR. CD74-positive macrophages were significantly reduced in preeclamptic placentas compared with controls. CD74-silenced macrophages showed that the adhesion molecules ALCAM, ICAM4, and Syndecan-2, as well as macrophage adhesion to trophoblasts were diminished. Naive and activated macrophages lacking CD74 showed a shift toward a proinflammatory signature with an increased secretion of tumor necrosis factor-α, chemokine (C-C motif) ligand 5, and monocyte chemotactic protein-1, when cocultured with trophoblasts compared with control macrophages. Trophoblasts stimulated by these factors express more CYP2J2, sFlt1, TNFα, and IL-8. CD74-knockout mice showed disturbed placental morphology, reduced junctional zone, smaller placentas, and impaired spiral artery remodeling with fetal growth restriction.

CONCLUSIONS

CD74 downregulation in placental macrophages is present in preeclampsia. CD74 downregulation leads to altered macrophage activation toward a proinflammatory signature and a disturbed crosstalk with trophoblasts.

The multifaceted roles of the invariant chain CD74--More than just a chaperone

The invariant chain (CD74) is well known for its essential role in antigen presentation by mediating assembly and subcellular trafficking of the MHCII complex. Beyond this, CD74 has also been implicated in a number of processes independent of MHCII. These include the regulation of endosomal trafficking, cell migration and cellular signalling as surface receptor of the pro-inflammatory cytokine macrophage migration inhibitory factor (MIF). In several forms of cancer, CD74 is up-regulated and associated with enhanced proliferation and metastatic potential. In this review, an overview of the diverse biological functions of the CD74 protein is provided with a particular focus on how these may be regulated. In particular, proteolysis of CD74 will be discussed as a central mechanism to control the actions of this important protein at different levels.

Variations in MHC Class II Antigen Processing and Presentation in Health and Disease

MHC class II (MHC-II) molecules are critical in the control of many immune responses. They are also involved in most autoimmune diseases and other pathologies. Here, we describe the biology of MHC-II and MHC-II variations that affect immune responses. We discuss the classic cell biology of MHC-II and various perturbations. Proteolysis is a major process in the biology of MHC-II, and we describe the various components forming and controlling this endosomal proteolytic machinery. This process ultimately determines the MHC-II-presented peptidome, including cryptic peptides, modified peptides, and other peptides that are relevant in autoimmune responses. MHC-II also variable in expression, glycosylation, and turnover. We illustrate that MHC-II is variable not only in amino acids (polymorphic) but also in its biology, with consequences for both health and disease.

Endogenous antigen processing drives the primary CD4+ T cell response to influenza

By convention, CD4⁺ T lymphocytes recognize foreign and self peptides derived from internalized antigens in combination with MHC class II molecules. Alternative pathways of epitope production have been identified but their contributions to host defense have not been established. We show here in a mouse infection model that the CD4⁺ T cell response to influenza, critical for durable protection from the virus, is driven principally by unconventional processing of antigen synthesized within the infected antigen-presenting cell, not by classical processing of endocytosed virions or material from infected cells. Investigation of the cellular components involved, including the H2-M molecular chaperone, the proteasome, and gamma-interferon inducible lysosomal thiol reductase revealed considerable heterogeneity in the generation of individual epitopes, an arrangement that ensures peptide diversity and broad CD4⁺ T cell engagement. These results could fundamentally revise strategies for rational vaccine design and may lead to key insights into the induction of autoimmune and anti-tumor responses.

Comparison of predicted and actual consequences of missense mutations

Each person's genome sequence has thousands of missense variants. Practical interpretation of their functional significance must rely on computational inferences in the absence of exhaustive experimental measurements. Here we analyzed the efficacy of these inferences in 33 de novo missense mutations revealed by sequencing in first-generation progeny of N-ethyl-N-nitrosourea-treated mice, involving 23 essential immune system genes. PolyPhen2, SIFT, MutationAssessor, Panther, CADD, and Condel were used to predict each mutation's functional importance, whereas the actual effect was measured by breeding and testing homozygotes for the expected in vivo loss-of-function phenotype. Only 20% of mutations predicted to be deleterious by PolyPhen2 (and 15% by CADD) showed a discernible phenotype in individual homozygotes. Half of all possible missense mutations in the same 23 immune genes were predicted to be deleterious, and most of these appear to become subject to purifying selection because few persist between separate mouse substrains, rodents, or primates. Because defects in immune genes could be phenotypically masked in vivo by compensation and environment, we compared inferences by the same tools with the in vitro phenotype of all 2,314 possible missense variants in TP53; 42% of mutations predicted by PolyPhen2 to be deleterious (and 45% by CADD) had little measurable consequence for TP53-promoted transcription. We conclude that for de novo or low-frequency missense mutations found by genome sequencing, half those inferred as deleterious correspond to nearly neutral mutations that have little impact on the clinical phenotype of individual cases but will nevertheless become subject to purifying selection.

Parasite Manipulation of the Invariant Chain and the Peptide Editor H2-DM Affects Major Histocompatibility Complex Class II Antigen Presentation during Toxoplasma gondii Infection

Toxoplasma gondii is an obligate intracellular protozoan parasite. This apicomplexan is the causative agent of toxoplasmosis, a leading cause of central nervous system disease in AIDS. It has long been known that T. gondii interferes with major histocompatibility complex class II (MHC-II) antigen presentation to attenuate CD4(+) T cell responses and establish persisting infections. Transcriptional downregulation of MHC-II genes by T. gondii was previously established, but the precise mechanisms inhibiting MHC-II function are currently unknown. Here, we show that, in addition to transcriptional regulation of MHC-II, the parasite modulates the expression of key components of the MHC-II antigen presentation pathway, namely, the MHC-II-associated invariant chain (Ii or CD74) and the peptide editor H2-DM, in professional antigen-presenting cells (pAPCs). Genetic deletion of CD74 restored the ability of infected dendritic cells to present a parasite antigen in the context of MHC-II in vitro. CD74 mRNA and protein levels were, surprisingly, elevated in infected cells, whereas MHC-II and H2-DM expression was inhibited. CD74 accumulated mainly in the endoplasmic reticulum (ER), and this phenotype required live parasites, but not active replication. Finally, we compared the impacts of genetic deletion of CD74 and H2-DM genes on parasite dissemination toward lymphoid organs in mice, as well as activation of CD4(+) T cells and interferon gamma (IFN-γ) levels during acute infection. Cyst burdens and survival during the chronic phase of infection were also evaluated in wild-type and knockout mice. These results highlight the fact that the infection is influenced by multiple levels of parasite manipulation of the MHC-II antigen presentation pathway.

Processing of CD74 by the Intramembrane Protease SPPL2a Is Critical for B Cell Receptor Signaling in Transitional B Cells

The invariant chain (CD74), a chaperone in MHC class II-mediated Ag presentation, is sequentially processed by different endosomal proteases. We reported recently that clearance of the final membrane-bound N-terminal fragment (NTF) of CD74 is mediated by the intramembrane protease signal peptide peptidase-like (SPPL)2a, a process critical for B cell development. In mice, SPPL2a deficiency provokes the accumulation of this NTF in endocytic vesicles, which leads to a B cell maturation arrest at the transitional 1 stage. To define the underlying mechanism, we analyzed the impact of SPPL2a deficiency on signaling pathways involved in B cell homeostasis. We demonstrate that tonic as well as BCR-induced activation of the PI3K/Akt pathway is massively compromised in SPPL2a(-/-) B cells and identify this as major cause of the B cell maturation defect in these mice. Altered BCR trafficking induces a reduction of surface IgM in SPPL2a-deficient B cells, leading to a diminished signal transmission via the BCR and the tyrosine kinase Syk. We provide evidence that in SPPL2a(-/-) mice impaired BCR signaling is to a great extent provoked by the accumulating CD74 NTF, which can interact with the BCR and Syk, and that impaired PI3K/Akt signaling and reduced surface IgM are not directly linked processes. In line with disturbances in PI3K/Akt signaling, SPPL2a(-/-) B cells show a dysregulation of the transcription factor FOXO1, causing elevated transcription of proapoptotic genes. We conclude that SPPL2a-mediated processing of CD74 NTF is indispensable to maintain appropriate levels of tonic BCR signaling to promote B cell maturation.

MHC Class II-restricted antigen presentation by plasmacytoid dendritic cells drives proatherogenic T cell immunity

BACKGROUND

Plasmacytoid dendritic cells (pDCs) bridge innate and adaptive immune responses and are important regulators of immuno-inflammatory diseases. However, their role in atherosclerosis remains elusive.

METHODS AND RESULTS

Here, we used genetic approaches to investigate the role of pDCs in atherosclerosis. Selective pDC deficiency in vivo was achieved using CD11c-Cre × Tcf4(-/flox) bone marrow transplanted into Ldlr(-/-) mice. Compared with control Ldlr(-/-) chimeric mice, CD11c-Cre × Tcf4(-/flox) mice had reduced atherosclerosis levels. To begin to understand the mechanisms by which pDCs regulate atherosclerosis, we studied chimeric Ldlr(-/-) mice with selective MHCII deficiency on pDCs. Significantly, these mice also developed reduced atherosclerosis compared with controls without reductions in pDC numbers or changes in conventional DCs. MHCII-deficient pDCs showed defective stimulation of apolipoprotein B100-specific CD4(+) T cells in response to native low-density lipoprotein, whereas production of interferon-α was not affected. Finally, the atheroprotective effect of selective MHCII deficiency in pDCs was associated with significant reductions of proatherogenic T cell-derived interferon-γ and lesional T cell infiltration, and was abrogated in CD4(+) T cell-depleted animals.

CONCLUSIONS

This study supports a proatherogenic role for pDCs in murine atherosclerosis and identifies a critical role for MHCII-restricted antigen presentation by pDCs in driving proatherogenic T cell immunity.

The D-6 mouse monoclonal antibody recognizes the CD74 cytoplasmic tail

The invariant chain (Ii; CD74) is a multifunctional protein of the immune system and a major player in the presentation of exogenous antigens to T cells. In the endoplasmic reticulum (ER), Ii assists the folding and trafficking of MHC class II molecules. In the present study, we characterized the recently commercialized D-6 monoclonal antibody (MAb) made against a polypeptide spanning the entire sequence of the p33 isoform of human Ii. Using transgenic mice expressing the human p35 isoform, we showed by flow cytometry that D-6 only slightly cross-reacts with mouse Ii in permeabilized splenocytes. Analysis of the human B lymphoblastoid cell line LG2 revealed that D-6 recognizes Ii only upon membrane permeabilization. Variants of Ii bearing specific mutations or deletions were transfected in human cells to map the D-6 epitope. Our results showed that this MAb binds to the N-terminal cytoplasmic domain of Ii and that the epitope was destroyed upon mutagenesis of the two leucine-based endosomal targeting motifs. Thus, D-6 cannot be used for rapid flow cytometric assessment of CD74 cell surface expression and would be ineffective as a drug conjugate for the treatment of hematological malignancies.

Boosting the MHC Class II-Restricted Tumor Antigen Presentation to CD4+ T Helper Cells: A Critical Issue for Triggering Protective Immunity and Re-Orienting the Tumor Microenvironment Toward an Anti-Tumor State

Although the existence of an immune response against tumor cells is well documented, the fact that tumors take off in cancer patients indicates that neoplastic cells can circumvent this response. Over the years many investigators have described strategies to rescue the anti-tumor immune response with the aim of creating specific and long-lasting protection against the disease. When exported to human clinical settings, these strategies have revealed in most cases a very limited, if any, positive outcome. We believe that the failure is mostly due to the inadequate triggering of the CD4+ T helper (TH) cell arm of the adaptive immunity, as TH cells are necessary to trigger all the immune effector mechanisms required to eliminate tumor cells. In this review, we focus on novel strategies that by stimulating MHC class II-restricted activation of TH cells generate a specific and persistent adaptive immunity against the tumor. This point is of critical importance for both preventive and therapeutic anti-tumor vaccination protocols, because adaptive immunity with its capacity to produce specific, long-lasting protection and memory responses is indeed the final goal of vaccination. We will discuss data from our as well as other laboratories which strongly suggest that triggering a specific and persistent anti-tumor CD4+ TH cell response stably modify not only the tumor microenvironment but also tumor-dependent extratumor microenvironments by eliminating and/or reducing the blood-derived tumor infiltrating cells that may have a pro-tumor growth function such as regulatory CD4+/CD25+ T cells and myeloid-derived-suppressor cells. Within this frame, therefore, we believe that the establishment of a pro-tumor environment is not the cause but simply the consequence of the tumor strategy to primarily counteract components of the adaptive cellular immunity, particularly TH lymphocytes.

A novel family of human leukocyte antigen class II receptors may have its origin in archaic human species

HLA class II α and β chains form receptors for antigen presentation to CD4(+) T cells. Numerous pairings of class II α and β subunits from the wide range of haplotypes and isotypes may form, but most of these combinations, in particular those produced by isotype mixing, yielded mismatched dimers. It is unclear how selection of functional receptors is achieved. At the atomic level, it is not known which interactions of class II residues regulate selection of matched αβ heterodimers and the evolutionary origin of matched isotype mixed dimer formation. In this study we investigated assembly of isotype-mixed HLA class II α and β heterodimers. Assembly and carbohydrate maturation of various HLA-class II isotype-mixed α and β subunits was dependent on the groove binding section of the invariant chain (Ii). By mutation of polymorphic DPβ sequences, we identified two motifs, Lys-69 and GGPM-(84-87), that are engaged in Ii-dependent assembly of DPβ with DRα. We identified five members of a family of DPβ chains containing Lys-69 and GGPM 84-87, which assemble with DRα. The Lys/GGPM motif is present in the DPβ sequence of the Neanderthal genome, and this ancient sequence is related to the human allele DPB1*0401. By site-directed mutagenesis, we inspected Neanderthal amino acid residues that differ from the DPB1*0401 allele and aimed to determine whether matched heterodimers are formed by assembly of DPβ mutants with DRα. Because the *0401 allele is rare in the sub-Saharan population but frequent in the European population, it may have arisen in modern humans by admixture with Neanderthals in Europe.

Exposing the Specific Roles of the Invariant Chain Isoforms in Shaping the MHC Class II Peptidome

The peptide repertoire (peptidome) associated with MHC class II molecules (MHCIIs) is influenced by the polymorphic nature of the peptide binding groove but also by cell-intrinsic factors. The invariant chain (Ii) chaperones MHCIIs, affecting their folding and trafficking. Recent discoveries relating to Ii functions have provided insights as to how it edits the MHCII peptidome. In humans, the Ii gene encodes four different isoforms for which structure-function analyses have highlighted common properties but also some non-redundant roles. Another layer of complexity arises from the fact that Ii heterotrimerizes, a characteristic that has the potential to affect the maturation of associated MHCIIs in many different ways, depending on the isoform combinations. Here, we emphasize the peptide editing properties of Ii and discuss the impact of the various isoforms on the MHCII peptidome.

MHC class II antigen presentation by dendritic cells regulated through endosomal sorting

For the initiation of adaptive immune responses, dendritic cells present antigenic peptides in association with major histocompatibility complex class II (MHCII) to naïve CD4(+) T lymphocytes. In this review, we discuss how antigen presentation is regulated through intracellular processing and trafficking of MHCII. Newly synthesized MHCII is chaperoned by the invariant chain to endosomes, where peptides from endocytosed pathogens can bind. In nonactivated dendritic cells, peptide-loaded MHCII is ubiquitinated and consequently sorted by the ESCRT machinery to intraluminal vesicles of multivesicular bodies, ultimately leading to lysosomal degradation. Ubiquitination of newly synthesized MHCII is blocked when dendritic cells are activated, now allowing its transfer to the cell surface. This mode of regulation for MHCII is a prime example of how molecular processing and sorting at multivesicular bodies can determine the expression of signaling receptors at the plasma membrane.

Virus-encoded ectopic CD74 enhances poxvirus vaccine efficacy

Vaccinia virus (VV) has been used globally as a vaccine to eradicate smallpox. Widespread use of this viral vaccine has been tempered in recent years because of its immuno-evasive properties, with restrictions prohibiting VV inoculation of individuals with immune deficiencies or atopic skin diseases. VV infection is known to perturb several pathways for immune recognition including MHC class II (MHCII) and CD1d-restricted antigen presentation. MHCII and CD1d molecules associate with a conserved intracellular chaperone, CD74, also known as invariant chain. Upon VV infection, cellular CD74 levels are significantly reduced in antigen-presenting cells, consistent with the observed destabilization of MHCII molecules. In the current study, the ability of sustained CD74 expression to overcome VV-induced suppression of antigen presentation was investigated. Viral inhibition of MHCII antigen presentation could be partially ameliorated by ectopic expression of CD74 or by infection of cells with a recombinant VV encoding murine CD74 (mCD74-VV). In contrast, virus-induced disruptions in CD1d-mediated antigen presentation persisted even with sustained CD74 expression. Mice immunized with the recombinant mCD74-VV displayed greater protection during VV challenge and more robust anti-VV antibody responses. Together, these observations suggest that recombinant VV vaccines encoding CD74 may be useful tools to improve CD4⁺ T-cell responses to viral and tumour antigens.

Macrophage migration inhibitory factor receptor CD74 mediates alphavirus-induced arthritis and myositis in murine models of alphavirus infection

OBJECTIVE

Arthrogenic alphaviruses such as Ross River virus (RRV) and chikungunya virus (CHIKV) circulate worldwide. This virus class causes debilitating illnesses that are characterized by arthritis, arthralgia, and myalgia. In previous studies, we identified macrophage migration inhibitory factor (MIF) as a critical inflammatory factor in the pathogenesis of alphaviral diseases. The present study was undertaken to characterize the role of CD74, a cell surface receptor of MIF, in both RRV- and CHIKV-induced alphavirus arthritides.

METHODS

Mouse models of RRV and CHIKV infection were used to investigate the immunopathogenesis of arthritic alphavirus infection. The role of CD74 was assessed using histologic analysis, real-time polymerase chain reaction, flow cytometry, and plaque assay.

RESULTS

In comparison to wild-type mice, CD74-/- mice developed only mild clinical features and had low levels of tissue damage. Leukocyte infiltration, characterized predominantly by inflammatory monocytes and natural killer cells, was substantially reduced in the infected tissue of CD74-/- mice, but production of proinflammatory cytokines and chemokines was not decreased. CD74 deficiency was associated with increased monocyte apoptosis, but had no effect on monocyte migratory capacity. Consistent with these findings, alphaviral infection resulted in a dose-dependent up-regulation of CD74 expression in human peripheral blood mononuclear cells, and serum MIF levels were significantly elevated in patients with RRV or CHIKV infection.

CONCLUSION

CD74 appears to regulate immune responses to alphaviral infection through its effects on cellular recruitment and survival. These findings suggest that both MIF and CD74 play a critical role in mediating alphaviral disease, and blocking these factors with novel therapeutic agents could substantially ameliorate the pathologic manifestations.

Deletion of CD74, a putative MIF receptor, in mice enhances osteoclastogenesis and decreases bone mass

CD74 is a type II transmembrane protein that can act as a receptor for macrophage migration inhibitory factor (MIF) and plays a role in MIF-regulated responses. We reported that MIF inhibited osteoclast formation and MIF knockout (KO) mice had decreased bone mass. We therefore examined if CD74 was involved in the ability of MIF to alter osteoclastogenesis in cultured bone marrow (BM) from wild-type (WT) and CD74-deficient (KO) male mice. We also measured the bone phenotype of CD74 KO male mice. Bone mass in the femur of 8-week-old mice was measured by micro-computed tomography and histomorphometry. Bone marrow cells from CD74 KO mice formed 15% more osteoclast-like cells (OCLs) with macrophage colony-stimulating factor (M-CSF) and receptor activator of NF-κB ligand (RANKL) (both at 30 ng/mL) compared to WT. Addition of MIF to WT cultures inhibited OCL formation by 16% but had no effect on CD74KO cultures. The number of colony forming unit granulocyte-macrophage (CFU-GM) in the bone marrow of CD74 KO mice was 26% greater than in WT controls. Trabecular bone volume (TBV) in the femurs of CD74 KO male mice was decreased by 26% compared to WT. In addition, cortical area and thickness were decreased by 14% and 11%, respectively. Histomorphometric analysis demonstrated that tartrate-resistant acid phosphatase (TRAP)(+) osteoclast number and area were significantly increased in CD74 KO by 35% and 43%, respectively compared to WT. Finally, we examined the effect of MIF on RANKL-induced-signaling pathways in bone marrow macrophage (BMM) cultures. MIF treatment decreased RANKL-induced nuclear factor of activated T cells, cytoplasmic 1 (NFATc1) and c-Fos protein in BMM cultures by 70% and 41%, respectively. Our data demonstrate that CD74 is required for MIF to affect in vitro osteoclastogenesis. Further, the bone phenotype of CD74 KO mice is similar to that of MIF KO mice. MIF treatment of WT cultures suppressed RANKL-induced activator protein 1 (AP-1) expression, which resulted in decreased osteoclast differentiation in vitro. We propose that CD74 plays a critical role in the MIF inhibition of osteoclastogenesis.

Pathways of antigen processing

T cell recognition of antigen-presenting cells depends on their expression of a spectrum of peptides bound to major histocompatibility complex class I (MHC-I) and class II (MHC-II) molecules. Conversion of antigens from pathogens or transformed cells into MHC-I- and MHC-II-bound peptides is critical for mounting protective T cell responses, and similar processing of self proteins is necessary to establish and maintain tolerance. Cells use a variety of mechanisms to acquire protein antigens, from translation in the cytosol to variations on the theme of endocytosis, and to degrade them once acquired. In this review, we highlight the aspects of MHC-I and MHC-II biosynthesis and assembly that have evolved to intersect these pathways and sample the peptides that are produced.

Pulse-chase analysis for studies of MHC class II biosynthesis, maturation, and peptide loading

Pulse-chase analysis is a commonly used technique for studying the synthesis, processing and transport of proteins. Cultured cells expressing proteins of interest are allowed to take up radioactively labeled amino acids for a brief interval ("pulse"), during which all newly synthesized proteins incorporate the label. The cells are then returned to nonradioactive culture medium for various times ("chase"), during which proteins may undergo conformational changes, trafficking, or degradation. Proteins of interest are isolated (usually by immunoprecipitation) and resolved by sodium dodecyl sulfate polyacrylamide gel electrophoresis (SDS-PAGE), and the fate of radiolabeled molecules is examined by autoradiography. This chapter describes a pulse-chase protocol suitable for studies of major histocompatibility complex (MHC) class II biosynthesis and maturation. We discuss how results are affected by the recognition by certain anti-class II antibodies of distinct class II conformations associated with particular biosynthetic states. Our protocol can be adapted to follow the fate of many other endogenously synthesized proteins, including viral or transfected gene products, in cultured cells.

The intramembrane protease SPPL2a promotes B cell development and controls endosomal traffic by cleavage of the invariant chain

The intramembrane protease SPPL2a cleaves the NTF of invariant chain (CD74), which is essential for normal trafficking of MHC class II–containing endosomes and thus for B cell development and function.

Regulated intramembrane proteolysis is a central cellular process involved in signal transduction and membrane protein turnover. The presenilin homologue signal-peptide-peptidase-like 2a (SPPL2a) has been implicated in the cleavage of type 2 transmembrane proteins. We show that the invariant chain (li, CD74) of the major histocompatability class II complex (MHCII) undergoes intramembrane proteolysis mediated by SPPL2a. B lymphocytes of SPPL2a^−/− mice accumulate an N-terminal fragment (NTF) of CD74, which severely impairs membrane traffic within the endocytic system and leads to an altered response to B cell receptor stimulation, reduced BAFF-R surface expression, and accumulation of MHCII in transitional developmental stage T1 B cells. This results in significant loss of B cell subsets beyond the T1 stage and disrupted humoral immune responses, which can be recovered by additional ablation of CD74. Hence, we provide evidence that regulation of CD74-NTF levels by SPPL2a is indispensable for B cell development and function by maintaining trafficking and integrity of MHCII-containing endosomes, highlighting SPPL2a as a promising pharmacological target for depleting and/or modulating B cells.