Sustained cross‐presentation capacity of murine splenic dendritic cell subsets in vivo

An exclusive feature of dendritic cells (DCs) is their ability to cross‐present exogenous antigens in MHC class I molecules. We analyzed the fate of protein antigen in antigen presenting cell (APC) subsets after uptake of naturally formed antigen‐antibody complexes in vivo. We observed that murine splenic DC subsets were able to present antigen in vivo for at least a week. After ex vivo isolation of four APC subsets, the presence of antigen in the storage compartments was visualized by confocal microscopy. Although all APC subsets stored antigen for many days, their ability and kinetics in antigen presentation was remarkably different. CD8α⁺ DCs showed sustained MHC class I‐peptide specific CD8⁺ T‐cell activation for more than 4 days. CD8α^? DCs also presented antigenic peptides in MHC class I but presentation decreased after 48 h. In contrast, only the CD8α^? DCs were able to present antigen in MHC class II to specific CD4⁺ T cells. Plasmacytoid DCs and macrophages were unable to activate any of the two T‐cell types despite detectable antigen uptake. These results indicate that naturally occurring DC subsets have functional antigen storage capacity for prolonged T‐cell activation and have distinct roles in antigen presentation to specific T cells in vivo.     

Selective antigen‐specific CD4+ T‐cell,but not CD8+ T‐ or B‐cell,tolerance corrupts cancer immunotherapy

Self‐tolerance, presumably through lineage‐unbiased elimination of self‐antigen‐specific lymphocytes (CD4⁺ T, CD8⁺ T, and B cells), creates a formidable barrier to cancer immunotherapy. In contrast to this prevailing paradigm, we demonstrate that for some antigens, self‐tolerance reflects selective elimination of antigen‐specific CD4⁺ T cells, but preservation of CD8⁺ T‐ and B‐cell populations. In mice, antigen‐specific CD4⁺ T‐cell tolerance restricted CD8⁺ T‐ and B‐cell responses targeting the endogenous self‐antigen guanylyl cyclase c (GUCY2C) in colorectal cancer. Although selective CD4⁺ T‐cell tolerance blocked GUCY2C‐specific antitumor immunity and memory responses, it offered a unique solution to the inefficacy of GUCY2C vaccines through recruitment of self‐antigen‐independent CD4⁺ T‐cell help. Incorporating CD4⁺ T‐cell epitopes from foreign antigens into vaccines against GUCY2C reconstituted CD4⁺ T‐cell help, revealing the latent functional capacity of GUCY2C‐specific CD8⁺ T‐ and B‐cell pools, producing durable antitumor immunity without autoimmunity. Incorporating CD4⁺ T‐cell epitopes from foreign antigens into vaccines targeting self‐antigens in melanoma (Trp2) and breast cancer (Her2) produced similar results, suggesting selective CD4⁺ T‐cell tolerance underlies ineffective vaccination against many cancer antigens. Thus, identification of self‐antigens characterized by selective CD4⁺ T‐cell tolerance and abrogation of such tolerance through self‐antigen‐independent T‐cell help is essential for future immunotherapeutics.     

Increased endogenous antigen presentation in the periphery enhances susceptibility to inflammation‐induced gastric autoimmunity in mice

How the immune system maintains peripheral tolerance under inflammatory conditions is poorly understood. Here we assessed the fate of gastritogenic T cells following inflammatory activation in vivo. Self‐reactive T cells (A23 T cells) specific for the gastric H⁺/K⁺ ATPase α subunit (HKα) were transferred into immunosufficient recipient mice and immunised at a site distant to the stomach with adjuvant containing the cognate HKα peptide antigen. Activation of A23 T cells by immunisation did not impact on either immune tolerance or protection from gastric autoimmunity in wild‐type BALB/c mice. However, increased presentation of endogenously derived HKα epitopes by dendritic cells (DCs) in the gastric lymph node of IE‐H⁺/K⁺β transgenic mice (IEβ) reduces A23 T‐cell tolerance to gastric antigens after inflammatory activation, with subsequent development of gastritis. While HKα‐specific A23 T cells from immunised wild‐type mice were poorly responsive to in vitro antigen specific activation, A23 T cells from immunised IEβ transgenic mice were readily re‐activated, indicating loss of T‐cell anergy. These findings show that DCs of gastric lymph nodes can maintain tolerance of pathogenic T cells following inflammatory stimulation and that the density of endogenous antigen presented to self‐reactive T cells is critical in the balance between tolerance and autoimmunity.     

Intrathecal CD4+ and CD8+ T‐cell responses to endogenously synthesized candidate disease‐associated human autoantigens in multiple sclerosis patients

MS pathology is potentially orchestrated by autoreactive T cells, but the antigens recognized remain unknown. A novel APC/T‐cell platform was developed to determine intrathecal CD4⁺ and CD8⁺ T‐cell responses to candidate MS‐associated autoantigens (cMSAg) in clinically isolated syndrome (CIS, n = 7) and MS (n = 6) patients. Human cMSAg encoding open reading frames (n = 8) were cloned into an Epstein–Barr virus (EBV)‐based vector to express cMSAg at high levels in EBV‐transformed B‐cells (BLCLs). Human cMSAg cloned were myelin‐associated and ‐oligodendrocyte glycoprotein, myelin basic protein, proteolipid protein, ATP‐dependent potassium channel ATP‐dependent inwards rectifying potassium channel 4.1, S100 calcium‐binding protein B, contactin‐2, and neurofascin. Transduced BLCLs were used as autologous APC in functional T‐cell assays to determine cMSAg‐specific T‐cell frequencies in cerebrospinal fluid derived T‐cell lines (CSF‐TCLs) by intracellular IFN‐γ flow cytometry. Whereas all CSF‐TCL responded strongly to mitogenic stimulation, no substantial T‐cell reactivity to cMSAg was observed. Contrastingly, measles virus fusion protein‐specific CD4⁺ and CD8⁺ T‐cell clones, used as control of the APC/T‐cell platform, efficiently recognized transduced BLCL expressing their cognate antigen. The inability to detect substantial T‐cell reactivity to eight human endogenously synthesized cMSAg in autologous APC do not support their role as prominent intrathecal T‐cell target antigens in CIS and MS patients early after onset of disease.     

ICAM‐1 and B7‐1 provide similar but distinct costimulation for CD8+ T cells,while CD4+ T cells are poorly costimulated by ICAM‐1

The function of purified ICAM‐1 in costimulating CD4⁺ and CD8⁺ T cell responses has been directly compared to that of B7‐1 in a model system that minimizes contributions of other receptor‐ligand interactions. While B7‐1 costimulates both subsets of T cells, ICAM‐1 is much more effective in the costimulation of CD8⁺ cells. ICAM‐1 also synergizes with B7‐1 for the induction of IL‐2 production in CD8⁺ but not CD4⁺ T cells. These differences are not explained by differences in LFA‐1 receptor expression on the two subsets of T cells. The CD8⁺ T cell response to ICAM‐1 costimulation is associated with increased proliferation and IL‐2 production at levels similar to those seen with B7‐1 costimulation, but clonal expansion in response to ICAM‐1 is not as great due to decreased cell survival. ICAM‐1‐mediated costimulation is effective for both naive and memory CT8⁺ T cells, is independent of CD28 engagement, and does not appear to be due solely to effects on adhesion. These results suggest that ICAM‐1‐dependent, B7‐independent costimulation may be important in initiating a CTL response to class I antigen presented by cells that are not professional APC.     

B7‐H1 and CD8+ Treg: The enigmatic role of B7‐H1 in peripheral tolerance

The interaction between B7‐H1 (PD‐L1) expressed on APC with PD‐1 expressed by T cells was shown previously to result in inhibition of T‐cell activation and autoimmune diseases. A paper in this issue of the European Journal of Immunology demonstrates that DC B7‐H1 expression can in fact enhance autoimmunity, rather than suppress it. Using a model of direct injection of self antigen‐loaded DC into the CNS, the authors demonstrate that DC with intact B7‐H1 expression exacerbate CNS autoimmune disease. Importantly, the improved disease outcome in animals treated with B7‐H1^?/? DC is a result of a population of CD8⁺ Treg cells that expand at the site of autoimmune inflammation.     

B cells expressing IFN‐γ suppress Treg‐cell differentiation and promote autoimmune experimental arthritis

Clinical efficacy in the treatment of rheumatoid arthritis with anti‐CD20 (Rituximab)‐mediated B‐cell depletion has garnered interest in the mechanisms by which B cells contribute to autoimmunity. We have reported that B‐cell depletion in a murine model of proteoglycan‐induced arthritis (PGIA) leads to an increase in Treg cells that correlate with decreased autoreactivity. Here, we demonstrate that the increase in Treg cells after B‐cell depletion is due to an increase in the differentiation of naïve CD4⁺ T cells into Treg cells. Since the development of PGIA is dependent on IFN‐γ and B cells are reported to produce IFN‐γ, we hypothesized that B‐cell‐specific IFN‐γ plays a role in the development of PGIA. Accordingly, mice with B‐cell‐specific IFN‐γ deficiency were as resistant to the induction of PGIA as mice that were completely IFN‐γ deficient. Importantly, despite a normal frequency of IFN‐γ‐producing CD4⁺ T cells, B‐cell‐specific IFN‐γ‐deficient mice exhibited a higher percentage of Treg cells compared with that in WT mice. These data indicate that B‐cell IFN‐γ production inhibits Treg‐cell differentiation and exacerbates arthritis. Thus, we have established that IFN‐γ, specifically derived from B cells, uniquely contributes to the pathogenesis of autoimmunity through prevention of immunoregulatory mechanisms.     

Memory B cells from a subset of treatment‐naïve relapsing‐remitting multiple sclerosis patients elicit CD4+ T‐cell proliferation and IFN‐γ production in response to myelin basic protein and myelin oligodendrocyte glycoprotein

Recent evidence suggests that B‐ and T‐cell interactions may be paramount in relapsing‐remitting MS (RRMS) disease pathogenesis. We hypothesized that memory B‐cell pools from RRMS patients may specifically harbor a subset of potent neuro‐APC that support neuro‐Ag reactive T‐cell proliferation and cytokine secretion. To test this hypothesis, we compared CD80 and HLA‐DR expression, IL‐10 and lymphotoxin‐α secretion, neuro‐Ag binding capacity, and neuro‐Ag presentation by memory B cells from RRMS patients to naïve B cells from RRMS patients and to memory and naïve B cells from healthy donors (HD). We identified memory B cells from some RRMS patients that elicited CD4⁺ T‐cell proliferation and IFN‐γ secretion in response to myelin basic protein and myelin oligodendrocyte glycoprotein. Notwithstanding the fact that the phenotypic parameters that promote efficient Ag presentation were observed to be similar between RRMS and HD memory B cells, a corresponding capability to elicit CD4⁺ T‐cell proliferation in response to myelin basic protein and myelin oligodendrocyte glycoprotein was not observed in HD memory B cells. Our results demonstrate for the first time that the memory B‐cell pool in RRMS harbors neuro‐Ag specific B cells that can activate T cells.     

Impaired B‐cell tolerance checkpoints promote the development of autoimmune diseases and pathogenic autoantibodies

A role for B cells in autoimmune diseases is now clearly established both in mouse models and humans by successful treatment of multiple sclerosis and rheumatoid arthritis with anti‐CD20 monoclonal antibodies that eliminate B cells. However, the underlying mechanisms by which B cells promote the development of autoimmune diseases remain poorly understood. Here, we review evidence that patients with autoimmune disease suffer from defects in early B‐cell tolerance checkpoints and therefore fail to counterselect developing autoreactive B cells. These B‐cell tolerance defects are primary to autoimmune diseases and may result from altered B‐cell receptor signaling and dysregulated T‐cell/regulatory T‐cell compartment. As a consequence, large numbers of autoreactive naive B cells accumulate in the blood of patients with autoimmune diseases and may promote autoimmunity through the presentation of self‐antigen to T cells. In addition, new evidence suggests that this reservoir of autoreactive naive B cells contains clones that may develop into CD27^?CD21^?/lo B cells associated with increased disease severity and plasma cells secreting potentially pathogenic autoantibodies after the acquisition of somatic hypermutations that improve affinity for self‐antigens.     

Antigen presentation by T cells: T cell receptor ligation promotes antigen acquisition from professional antigen-presenting cells

The purpose of this study was to determine whether the clonotypic specificity of the T cell receptor influences the specificity of T cell-mediated antigen presentation. We have previously shown that myelin basic protein (MBP)-specific Lewis rat GP2.E5/R1 (R1) T cells cultured with antigen, irradiated syngeneic splenocytes (IrrSPL) and tolerogenic monoclonal antibody become highly effective antigen-presenting cells (APC). In the current studies, we investigated the transfer of specific (MBP) and unrelated (conalbumin) antigens from antigen-pulsed SPL to R1 T cells. R1 T cells cultured with IrrSPL that were pulsed simultaneously with both MBP and conalbumin acquired and presented both antigens to the appropriate T cell responders in a secondary assay. These results suggested a physical transfer of major histocompatibility complex (MHC)/peptide complexes from professional APC to R1 T cells. Transfer of conalbumin from professional APC to R1 T cells required specific recognition of MBP and was optimal when both conalbumin and MBP were presented on the same group of professional APC. Antigens transfer did not occur when allogeneic SPL were used as APC. The anti-I-A mAb OX6 inhibited antigen transfer but only when added during the initiation of culture. OX6 also inhibited antigen acquisition by R1-trans, a variant of the R1 T cell line which constitutively synthesizes high levels of I-A, from MBP-pulsed IrrSPL but blockade of I-A did not inhibit antigen acquisition when soluble MBP was added directly to the culture. Despite constitutive synthesis of I-A, R1-trans T cells did not acquire guinea pig MBP from pulsed allogeneic APC. These studies demonstrate that although T cells of a particular specificity can present unrelated antigens, the cognate interaction of the T cell antigen receptor with the appropriate antigen/self-MHC complex strongly promotes acquisition of these complexes from professional APC.     

Involvement of CD91 and scavenger receptors in Hsp70‐facilitated activation of human antigen‐specific CD4+ memory T cells

Hsp70 plays several roles in the adaptive immune response. Based on the ability to interact with diverse peptides, extracellular Hsp70:peptide complexes exert profound effects both in autoimmunity and in tumor rejection by evoking potent T cell responses to the chaperoned peptide. The interaction with receptors on APC represents the basis for the immunological functions of Hsp70 and a critical point where the immune response can be regulated. Various surface proteins (e.g. CD91, scavenger receptors (SR)) have been implicated in binding of Hsp70. In this study, antigenic peptides from tetanus toxin and influenza hemagglutinin complexed to human stress‐inducible Hsp70 were found to enhance the proliferation and cytokine production of human antigen‐specific CD4⁺ T cells. This was demonstrated in proliferation experiments using human monocytes as APC. Proliferated antigen‐specific cells were detected combining HLA‐DRB1^*0401 or HLA‐DRB1^*1101 tetramer and CFSE staining. Treating monocytes with CD91 siRNA diminished these effects. Additional blocking of SR by the SR ligand fucoidan completely abolished enhanced proliferation and production of Th1 and Th2 cytokines. Taken together, our data indicate that in the human system, CD91 and members of the SR family efficiently direct Hsp70:peptide complexes into the MHC class II presentation pathway and thus enhance antigen‐specific CD4⁺ T cell responses.     

Opposing effects of actin signaling and LFA‐1 on establishing the affinity threshold for inducing effector T‐cell responses in mice

Mature CD8⁺ T cells use a narrow antigen affinity threshold to generate tissue‐infiltrating cytotoxic effector T cells and induce autoimmune pathology, but the mechanisms that establish this antigen affinity threshold are poorly understood. Only antigens with affinities above the threshold induce stable contacts with APCs, polarization of a T cell, and asymmetric T‐cell division. Previously published data indicate that LFA‐1 inside‐out signaling might be involved in establishing the antigen affinity threshold. Here, we show that subthreshold antigens weakly activate all major distal TCR signaling pathways. Low‐affinity antigens are more dependent on LFA‐1 than suprathreshold antigens. Moreover, augmenting the inside‐out signaling by hyperactive Rap1 does not increase responses to the subthreshold antigens. Thus, LFA‐1 signaling does not contribute to the affinity‐based antigen discrimination. However, we found that subthreshold antigens do not induce actin rearrangement toward an APC, mediated by Rho‐family GTPases, Cdc42, and Rac. Our data suggest that Rac and Cdc42 contribute to the establishment of the antigen affinity threshold in CD8⁺ T cells by enhancing responses to high‐affinity antigens, or by reducing the responses to low‐affinity antigens.     

Insulinoma‐released exosomes activate autoreactive marginal zone‐like B cells that expand endogenously in prediabetic NOD mice

Exosomes (EXOs) are nano‐sized secreted microvesicles that can function as potent endogenous carriers of adjuvant and antigens. To examine a possible role in autoimmunity for EXOs, we studied EXO‐induced immune responses in nonobese diabetic (NOD) mice, an autoimmune‐prone strain with tissue‐specific targeting at insulin‐secreting beta cells. EXOs released by insulinoma cells can activate various antigen‐presenting cells to secrete several proinflammatory cytokines and chemokines. A subset of B cells responded to EXO stimulation in culture by proliferation, and expressed surface markers representing marginal zone B cells, which was independent of T helper cells. Importantly, splenic B cells from prediabetic NOD mice, but not diabetic‐resistant mice, exhibited increased reactivity to EXOs, which was correlated with a high level of serum EXOs. We found that MyD88‐mediated innate TLR signals were essential for the B‐cell response; transgenic B cells expressing surface immunoglobulin specific for insulin reacted to EXO stimulation, and addition of a calcineurin inhibitor FK506 abrogated the EXO‐induced B‐cell response, suggesting that both innate and antigen‐specific signals may be involved. Thus, EXOs may contribute to the development of autoimmunity and type 1 diabetes in NOD mice, partially via activating autoreactive marginal zone‐like B cells.     

Impaired cross‐presentation of CD8α+CD11c+ dendritic cells by Japanese encephalitis virus in a TLR2/MyD88 signal pathway‐dependent manner

Cross‐presentation is the pathway by which exogenous antigens are routed for presentation by MHC class I molecules leading to activation of antiviral CD8⁺ T‐cell responses. However, there is little information describing the modulation of cross‐presentation and the impact of pathogen‐derived signals associated with Japanese encephalitis virus (JEV), which is one of the most common causes of encephalitis in humans. In this study, we demonstrate that JEV infection could suppress in vivo cross‐presentation of soluble and cell‐associated antigens, thereby generating weak CD8⁺ T‐cell responses to exogenous antigens, as evaluated by CFSE dilution of adoptively transferred CD8⁺ T cells and in vivo CTL killing activity. Furthermore, CD8α⁺CD11c⁺ dendritic cells (DCs), which are known to be far more efficient at cross‐presenting soluble antigens, played a specific role in contributing to JEV‐mediated inhibition of the cross‐presentation of exogenous antigens through interference with effective antigen uptake. Finally, this study provides evidence that TLR2‐MyD88 and p38 MAPK signal pathway might be involved in JEV‐mediated inhibition of cross‐presentation of soluble and cell‐associated antigens. These observations suggest that the modulation of cross‐presentation of exogenous antigens through TLR signaling has important implications for antiviral immune responses against JEV infection and the development of effective vaccination strategies.     

CD25+ Treg specifically suppress auto‐Ab generation against pancreatic tissue autoantigens

To study B‐cell tolerance against non‐lymphoid tissue autoantigens, we generated transgenic rat insulin promoter (RIP)‐OVA/hen egg lysozyme (HEL) mice expressing the model antigens, OVA and HEL, in pancreatic islets. Their vaccination with OVA or HEL induced far less auto‐Ab titers compared with non‐transgenic controls. Depletion of CD25⁺ cells during immunization completely restored auto‐Ab production, but did not affect antibodies against a foreign control antigen. Depletion at later time‐points was not effective. OVA‐specific CD25⁺ FoxP3⁺ T_reg were more frequent in the autoantigen‐draining pancreatic LN than in other secondary lymphatics of RIP‐OVA/HEL mice. Consistently, B cells were suppressed in that LN and also in the spleen, which is known to concentrate circulating antigen, such as the antigens used for vaccination. Suppression involved preventing expansion of autoreactive B cells in response to autoantigen, reducing antibody production per B‐cell and isotype changes. These findings demonstrate that CD25⁺ T_reg suppress auto‐Ab production against non‐lymphoid tissue antigens in an antigen‐specific manner.     

Mature proteins derived from Epstein–Barr virus fail to feed into the MHC class I antigenic pool

The immediate presentation of peptide epitopes on MHC class I (MHC I) after antigen expression has led to the concept that MHC I ligands are mostly derived from defective ribosomal products (DRiPs), a subset of newly synthesized proteins that are rapidly degraded by the proteasome. Whether and to what extent mature proteins contribute to the antigenic pool, however, has remained elusive. Here, we developed a conditional antigen expression system that allows studying antigen presentation from mature proteins by inducing their rapid proteasomal degradation in the absence of further antigen synthesis. Target cells in which expression of two Epstein–Barr virus (EBV) antigens was induced were rapidly recognized by antigen‐specific CD8⁺ T cells in a time‐ and dosage‐dependent manner, demonstrating that antigen presentation was linked to antigen synthesis. By contrast, T cells failed to recognize target cells containing large amounts of mature protein even after induction of their rapid proteasomal degradation. Thus, the presentation of these antigens proved to be strictly dependent on protein synthesis whereas mature proteins failed to furnish the antigenic pool. These results have implications for the design of immunotherapeutic strategies that aim at targeting proteins with increased half‐lives and are hence overexpressed in tumors.     

Reassessing the role of HLA‐DRB3 T‐cell responses: Evidence for significant expression and complementary antigen presentation

In humans, several HLA‐DRB loci (DRB1/3/4/5) encode diverse β‐chains that pair with α‐chains to form DR molecules on the surface of APC. While DRB1 and DRB5 have been extensively studied, the role of DRB3/4 products of DR52/DR53 haplotypes has been largely neglected. To clarify the relative expression of DRB3, we quantified DRB3 mRNA levels in comparison with DRB1 mRNA from the same haplotype in both B cells and monocytes, observing quantitatively significant DRB3 synthesis. In CD19⁺ cells, DRB1*03/11/13 was 3.5‐fold more abundant than DRB3, but in CD14⁺ this difference was only two‐fold. Monocytes also had lower overall levels of DR mRNA compared with B cells, which was confirmed by cell surface staining of DRB1 and DRB3. To evaluate the functional role of DRB3, tetramer‐guided epitope mapping was used to detect T cells against tetanus toxin and several influenza antigens presented by DRB3*0101/0202 or DRB1*03/11/13. None of the epitopes discovered were shared among any of the DR molecules. Quantitative assessment of DRB3‐tetanus toxin specific T cells revealed that they are present at similar frequencies as those observed for DRB1. These results suggest that DRB3 plays a significant role in antigen presentation with different epitopic preferences to DRB1. Therefore, DRB3, like DRB5, serves to extend and complement the peptide repertoire of DRB1 in antigen presentation.     

TGF‐β‐producing regulatory B cells induce regulatory T cells and promote transplantation tolerance

Regulatory B (Breg) cells have been shown to play a critical role in immune homeostasis and in autoimmunity models. We have recently demonstrated that combined anti‐T cell immunoglobulin domain and mucin domain‐1 and anti‐CD45RB antibody treatment results in tolerance to full MHC‐mismatched islet allografts in mice by generating Breg cells that are necessary for tolerance. Breg cells are antigen‐specific and are capable of transferring tolerance to untreated, transplanted animals. Here, we demonstrate that adoptively transferred Breg cells require the presence of regulatory T (Treg) cells to establish tolerance, and that adoptive transfer of Breg cells increases the number of Treg cells. Interaction with Breg cells in vivo induces significantly more Foxp3 expression in CD4⁺CD25^? T cells than with naive B cells. We also show that Breg cells express the TGF‐β associated latency‐associated peptide and that Breg‐cell mediated graft prolongation post‐adoptive transfer is abrogated by neutralization of TGF‐β activity. Breg cells, like Treg cells, demonstrate preferential expression of both C‐C chemokine receptor 6 and CXCR3. Collectively, these findings suggest that in this model of antibody‐induced transplantation tolerance, Breg cells promote graft survival by promoting Treg‐cell development, possibly via TGF‐β production.     

Ribosomal versus non-ribosomal cellular antigens: factors determining efficiency of indirect presentation to CD4+ T cells

Proteins released from dying cells can be taken up and presented by antigen‐presenting cells (APC) to T cells. While the presentation of such self antigens may lead to beneficial anti‐tumour responses, in autoimmune disease it leads to pathological immune responses. The sub‐set of self proteins targeted in autoimmune disease is circumscribed, and certain cellular components such as ribonucleoprotein (RNP) complexes are often targeted. Although explanations for this antigen selectivity have been proposed, there has been little direct testing of these hypotheses. We and others previously showed that ribosomal proteins, targeted in autoimmune disease, are also targets of anti‐tumour T‐cell responses. We asked whether particular properties of ribosomal proteins such as incorporation into RNP complexes or sub‐cellular localization enhance ribosomal protein presentation by APC to CD4⁺ T cells. Ribosomal protein antigens within purified intact ribosomes or free of the ribosomes were equally well taken up and presented by APC, demonstrating that inclusion of ribosomal proteins into an RNP complex does not confer an advantage. However, antigens localized to ribosomes within apoptotic cells were less efficiently taken up and presented by APC than the same antigens localized diffusely throughout the cell. This suggests that presentation of ribosomal proteins is somehow down‐regulated, possibly to facilitate presentation of other less‐abundant intracellular proteins. Consequently, the explanation for the frequent targeting of ribosomal proteins by both autoimmune and anti‐tumour T‐cell responses is not at the level of uptake from apoptotic cells and must be sought elsewhere.