RASGRP1 deficiency causes immunodeficiency with impaired cytoskeletal dynamics.

RASGRP1 is an important guanine nucleotide exchange factor and activator of the RAS-MAPK pathway following T cell antigen receptor (TCR) signaling. The consequences of RASGRP1 mutations in humans are unknown. In a patient with recurrent bacterial and viral infections, born to healthy consanguineous parents, we used homozygosity mapping and exome sequencing to identify a biallelic stop-gain variant in RASGRP1. This variant segregated perfectly with the disease and has not been reported in genetic databases. RASGRP1 deficiency was associated in T cells and B cells with decreased phosphorylation of the extracellular-signal-regulated serine kinase ERK, which was restored following expression of wild-type RASGRP1. RASGRP1 deficiency also resulted in defective proliferation, activation and motility of T cells and B cells. RASGRP1-deficient natural killer (NK) cells exhibited impaired cytotoxicity with defective granule convergence and actin accumulation. Interaction proteomics identified the dynein light chain DYNLL1 as interacting with RASGRP1, which links RASGRP1 to cytoskeletal dynamics. RASGRP1-deficient cells showed decreased activation of the GTPase RhoA. Treatment with lenalidomide increased RhoA activity and reversed the migration and activation defects of RASGRP1-deficient lymphocytes.

The energy sensor AMPK orchestrates metabolic and translational adaptation in expanding T helper cells.

The importance of cellular metabolic adaptation in inducing robust T cell responses is well established. However, the mechanism by which T cells link information regarding nutrient supply to clonal expansion and effector function is still enigmatic. Herein, we report that the metabolic sensor adenosine monophosphate-activated protein kinase (AMPK) is a critical link between cellular energy demand and translational activity and, thus, orchestrates optimal expansion of T cells in vivo. AMPK deficiency did not affect T cell fate decision, activation, or T effector cell generation; however, the magnitude of T cell responses in murine in vivo models of T cell activation was markedly reduced. This impairment was global, as all T helper cell subsets were similarly sensitive to loss of AMPK which resulted in reduced T cell accumulation in peripheral organs and reduced disease severity in pathophysiologically as diverse models as T cell transfer colitis and allergic airway inflammation. T cell receptor repertoire analysis confirmed similar clonotype frequencies in different lymphoid organs, thereby supporting the concept of a quantitative impairment in clonal expansion rather than a skewed qualitative immune response. In line with these findings, in-depth metabolic analysis revealed a decrease in T cell oxidative metabolism, and gene set enrichment analysis indicated a major reduction in ribosomal biogenesis and mRNA translation in AMPK-deficient T cells. We, thus, provide evidence that through its interference with these delicate processes, AMPK orchestrates the quantitative, but not the qualitative, manifestation of primary T cell responses in vivo.

TIM-3 and CEACAM1 do not interact in cis and in trans.

TIM-3 has been considered as a target in cancer immunotherapy. In T cells, inhibitory as well as activating functions have been ascribed to this molecule. Its role may therefore depend on the state of T cells and on the presence of interaction partners capable to perform functional pairing. Carcinoembryonic antigen-related cell adhesion molecule (CEACAM1) has been proposed to bind TIM-3 and to regulate its function. Using a T cell reporter platform we confirmed CEACAM1-mediated inhibition, but CEACAM1 did not functionally engage TIM-3. TIM-3 and CEACAM1 coexpression was limited to a small subset of activated T cells. Moreover, results obtained in extensive binding studies were not in support of an interaction between TIM-3 and CEACAM1. Cytoplasmic sequences derived from TIM-3 induced inhibitory signaling in our human T cell reporter system. Our results indicate that TIM-3 functions are independent of CEACAM1 and that this receptor has the capability to promote inhibitory signaling pathways in human T cells.

Rhinovirus induces an anabolic reprogramming in host cell metabolism essential for viral replication.

Rhinoviruses (RVs) are responsible for the majority of upper airway infections; despite their high prevalence and the resulting economic burden, effective treatment is lacking. We report here that RV induces metabolic alterations in host cells, which offer an efficient target for antiviral intervention. We show that RV-infected cells rapidly up-regulate glucose uptake in a PI3K-dependent manner. In parallel, infected cells enhance the expression of the PI3K-regulated glucose transporter GLUT1. In-depth metabolomic analysis of RV-infected cells revealed a critical role of glucose mobilization from extracellular and intracellular pools via glycogenolysis for viral replication. Infection resulted in a highly anabolic state, including enhanced nucleotide synthesis and lipogenesis. Consistently, we observed that glucose deprivation from medium and via glycolysis inhibition by 2-deoxyglucose (2-DG) potently impairs viral replication. Metabolomic analysis showed that 2-DG specifically reverts the RV-induced anabolic reprogramming. In addition, treatment with 2-DG inhibited RV infection and inflammation in a murine model. Thus, we demonstrate that the specific metabolic fingerprint of RV infection can be used to identify new targets for therapeutic intervention.

T-cell-derived cytokines enhance the antigen-presenting capacity of human neutrophils.

Activated allergen-specific Th2 and Th1 cells release cytokines that transform neutrophils into functional APCs characterized by the expression of HLA-DR and CD58 as well as enhanced survival and antigen uptake, irrespectively of the presence of IL-10, which reduces allergen uptake by neutrophils.

A novel role for neutrophils in IgE-mediated allergy: Evidence for antigen presentation in late-phase reactions.

BACKGROUND: Neutrophils and allergen-specific T cells accumulate in patients with allergic late-phase reactions (LPRs). Their presence is associated with severe inflammation. Cytokines, such as GM-CSF, IFN-γ, and IL-3, which are typically found in patients with allergic LPRs, have been proposed to convert neutrophils into antigen-presenting cells (APCs). OBJECTIVE: We sought to assess the antigen-processing and antigen-presenting capacities of neutrophils from allergic patients. METHODS: Neutrophils were isolated from peripheral blood of donors with birch pollen allergy and stimulated with GM-CSF, IFN-γ, and IL-3. The viability and expression of HLA-DR, CD80, and CD86 were assessed by using flow cytometry. HLA-DM expression was analyzed by means of immunoblotting. Allergen uptake was studied after fluorescence labeling of the major birch pollen allergen Bet v 1. Bet v 1 was digested with neutrophilic endolysosomal extracts, and the resulting fragments were sequenced by using mass spectrometry. Neutrophils were used as APCs in coculture experiments with autologous HLA-DR-restricted and Bet v 1-specific T-cell clones reactive with epitopes in different regions of the allergen. In all experiments monocytes were used for comparison. Fluids from suction blisters formed on top of LPRs induced by using intradermal allergen injection were assessed for HLA-DR+ neutrophils by using flow cytometry. RESULTS: The cytokines significantly enhanced the survival, allergen uptake, and expression of HLA-DM and HLA-DR on neutrophils. Neutrophils rapidly degraded Bet v 1 into fragments containing all relevant T-cell epitopes. Cytokine-activated, allergen-pulsed neutrophils induced proliferative and cytokine responses of Bet v 1-specific T cells irrespective of epitope specificity, confirming that they fully processed and presented the allergen. HLA-DR+ neutrophils were detected in patients with cutaneous allergic LPRs. CONCLUSION: Neutrophils can serve as APCs for local allergen-specific effector T cells in patients with allergic LPRs.

CTLA-4 antibody ipilimumab negatively affects CD4+ T-cell responses in vitro.

Immune checkpoint inhibitors targeting coinhibitory pathways in T cells possess efficacy in combating cancer. In addition to PD-1/PD-L1 and CTLA-4 antibodies which are already established in tumor immunotherapy, immune checkpoints such as LAG-3 or BTLA are emerging, which may have the potential to enhance T-cell responses alone or in combination with PD-1 blockers. CD4+ T cells play a central role in the immune system and contribute to productive immune responses in multiple ways. The effects of immune checkpoint inhibitors on this cell subset may thus critically influence therapeutic outcomes. Here, we have used in vitro responses to tetanus toxoid (TT) as a model system to study the effects of immune checkpoint inhibitors on CD4+ T-cell responses. CFSE-labeled PBMCs of 65 donors were stimulated with TT in the presence of blocking antibodies to PD-L1, CTLA-4, LAG-3, or BTLA for 7 days. We found that the PD-L1 antibody greatly enhanced cytokine production and antigen-specific CD4+ T-cell proliferation, whereas blocking antibodies to BTLA or LAG-3 did not augment responses to TT. Surprisingly, the presence of the therapeutic CTLA-4 antibody ipilimumab resulted in a significant reduction of CD4+ T-cell proliferation and cytokine production. Stimulation experiments with an IgG4 variant of ipilimumab indicated that the inhibitory effect of ipilimumab was dependent on its IgG1 isotype. Our results indicate that the therapeutic CTLA-4 antibody ipilimumab can impair CD4+ effector T-cell responses and that this activity is mediated by its Fc part and CD16-expressing cells.

Fusion proteins of flagellin and the major birch pollen allergen Bet v 1 show enhanced immunogenicity, reduced allergenicity, and intrinsic adjuvanticity.

BACKGROUND: Recombinant fusion proteins of flagellin and antigens have been demonstrated to induce strong innate and adaptive immune responses. Such fusion proteins can enhance the efficacy of allergen-specific immunotherapy. OBJECTIVE: We sought to characterize different fusion proteins of flagellin and the major birch pollen allergen Bet v 1 for suitability as allergy vaccines. METHODS: A truncated version of flagellin (NtCFlg) was genetically fused to the N- or C-terminus of Bet v 1. Toll-like receptor (TLR) 5 binding was assessed with HEK293 cells expressing TLR5. Upregulation of CD40, CD80, CD83, and CD86 on monocyte-derived dendritic cells from allergic patients was analyzed by using flow cytometry. The T cell-stimulatory capacity of the fusion proteins was assessed with naive and Bet v 1-specific T cells. IgE binding was tested in inhibition ELISAs and basophil activation tests. Mice were immunized with the fusion proteins in the absence and presence of aluminum hydroxide. Cellular and antibody responses were monitored. Murine antibodies were tested for blocking capacity in basophil activation tests. RESULTS: Both fusion proteins matured monocyte-derived dendritic cells through TLR5. Compared with Bet v 1, the fusion proteins showed stronger T cell-stimulatory and reduced IgE-binding capacity and induced murine Bet v 1-specific antibodies in the absence of aluminum hydroxide. However, only antibodies induced by means of immunization with NtCFlg fused to the C-terminus of Bet v 1 inhibited binding of patients' IgE antibodies to Bet v 1. CONCLUSION: Bet v 1-flagellin fusion proteins show enhanced immunogenicity, reduced allergenicity, and intrinsic adjuvanticity and thus represent promising vaccines for birch pollen allergen-specific immunotherapy. However, the sequential order of allergen and adjuvant within a fusion protein determines its immunologic characteristics.

The soluble cytoplasmic tail of CD45 (ct-CD45) in human plasma contributes to keep T cells in a quiescent state.

The cytoplasmic tail of CD45 (ct-CD45) is proteolytically cleaved and released upon activation of human phagocytes. It acts on T cells as an inhibitory, cytokine-like factor in vitro. Here, we show that ct-CD45 is abundant in human peripheral blood plasma from healthy adults compared with plasma derived from umbilical cord blood and plasma from patients with rheumatoid arthritis or systemic lupus erythematosus. Plasma depleted of ct-CD45 enhanced T-cell proliferation, while addition of exogenous ct-CD45 protein inhibited proliferation and reduced cytokine production of human T lymphocytes in response to TCR signaling. Inhibition of T-cell proliferation by ct-CD45 was overcome by costimulation via CD28. T-cell activation in the presence of ct-CD45 was associated with an upregulation of the quiescence factors Schlafen family member 12 (SLFN12) and Krueppel-like factor 2 (KLF2) as well as of the cyclin-dependent kinase (CDK) inhibitor p27kip1. In contrast, positive regulators of the cell cycle such as cyclin D2 and D3 as well as CDK2 and CDK4 were found to be downregulated in response to ct-CD45. In summary, we demonstrate that ct-CD45 is present in human plasma and sets the threshold of T-cell activation.

Association of CD147 and Calcium Exporter PMCA4 Uncouples IL-2 Expression from Early TCR Signaling.

The Ig superfamily member CD147 is upregulated following T cell activation and was shown to serve as a negative regulator of T cell proliferation. Thus, Abs targeting CD147 are being tested as new treatment strategies for cancer and autoimmune diseases. How CD147 mediates immunosuppression and whether association with other coreceptor complexes is needed have remained unknown. In the current study, we show that silencing of CD147 in human T cells increases IL-2 production without affecting the TCR proximal signaling components. We mapped the immunosuppressive moieties of CD147 to its transmembrane domain and Ig-like domain II. Using affinity purification combined with mass spectrometry, we determined the domain specificity of CD147 interaction partners and identified the calcium exporter plasma membrane calcium ATPase isoform 4 (PMCA4) as the interaction partner of the immunosuppressive moieties of CD147. CD147 does not control the proper membrane localization of PMCA4, but PMCA4 is essential for the CD147-dependent inhibition of IL-2 expression via a calcium-independent mechanism. In summary, our data show that CD147 interacts via its immunomodulatory domains with PMCA4 to bypass TCR proximal signaling and inhibit IL-2 expression.

Antibodies targeting BTLA or TIM-3 enhance HIV-1 specific T cell responses in combination with PD-1 blockade.

Persistent stimulation with antigens derived from viruses that establish chronic infections or tumour antigens results in the exhaustion of T cells. Coinhibitory receptors like PD-1 and CTLA-4 function as immune checkpoints on exhausted T cells. Blocking these molecules with antibodies improve immunity to cancer cells. Immune checkpoint inhibitors targeting other coinhibitory receptors might have a similar role in improving T cell function and thus also utility in cancer therapy. Using HIV-specific T cells as a model for exhaustion we have evaluated the capacity of antibodies targeting TIM-3, BTLA, CD160, LAG-3 and CTLA-4 alone or in combination with a PD-1 antibody to enhance proliferation and cytokine production in response to Gag and Nef peptides. Antibodies targeting BTLA and TIM-3 enhanced CD8 T cell proliferation. Moreover, our results indicate that blocking BTLA and TIM-3 in combination with PD-1 might be especially effective in enhancing responses of exhausted human T cells.

The AMP analog AICAR modulates the Treg/Th17 axis through enhancement of fatty acid oxidation.

T cells must tightly regulate their metabolic processes to cope with varying bioenergetic demands depending on their state of differentiation. The metabolic sensor AMPK is activated in states of low energy supply and modulates cellular metabolism toward a catabolic state. Although this enzyme is known to be particularly active in regulatory T (Treg) cells, its impact on T helper (Th)-cell differentiation is poorly understood. We investigated the impact of several AMPK activators on Treg-cell differentiation and found that the direct activator AICAR (5-aminoimidazole-4-carboxamide ribonucleotide), but not the indirect activators metformin and 2-deoxyglucose, strongly enhanced Treg-cell induction by specifically enhancing Treg-cell expansion. Conversely, Th17 generation was impaired by the agent. Further investigation of the metabolic background of our observations revealed that AICAR enhanced both cellular mitochondrogenesis and fatty acid uptake. Consistently, increased Treg induction was entirely reversible on inhibition of fatty acid oxidation, thus confirming the dependence of AICAR's effects on metabolic pathways alterations. Translating our findings to an in vivo model, we found that the substance enhanced Treg cell generation on IL-2 complex-induced immune stimulation. We provide a previously unrecognized insight into the delicate interplay between immune cell function and metabolism and delineate a potential novel strategy for metabolism-targeting immunotherapy.-Gualdoni, G. A., Mayer, K. A., Göschl, L., Boucheron, N., Ellmeier, W., Zlabinger, G. J. The AMP analog AICAR modulates the Treg/Th17 axis through enhancement of fatty acid oxidation.

Ig-like transcript 4 as a cellular receptor for soluble complement fragment C4d.

Complement regulation leads to the generation of complement split products (CSPs) such as complement component (C)4d, a marker for disease activity in autoimmune syndromes or antibody-mediated allograft rejection. However, the physiologic role of C4d has been unknown. By screening murine thymoma BW5147 cells expressing a cDNA library generated from human monocyte-derived dendritic cells with recombinant human C4d, we identified Ig-like transcript (ILT)4 and ILT5v2 as cellular receptors for C4d. Both receptors, expressed on monocytes, macrophages, and dendritic cells, also interacted with the CSPs C3d, C4b, C3b, and iC3b. However, C4d did not bind to classic complement receptors (CRs). Interaction between cell surface-resident ILT4 and soluble monomeric C4d resulted in endocytosis of C4d. Surprisingly, binding of soluble ILT4 to C4d covalently immobilized to a cellular surface following classic complement activation could not be detected. Remarkably, C4d immobilized to a solid phaseviaits intrinsic thioester conferred a dose-dependent inhibition of TNF-α and IL-6 secretion in monocytes activatedviaFc-cross-linking of up to 50% as compared to baseline. Similarly, C4d conferred an attenuation of intracellular Ca(2+)flux in monocytes activatedviaFc-cross-linking. In conclusion, ILT4 represents a scavenger-type endocytotic CR for soluble monomeric C4d, whereas attenuation of monocyte activation by physiologically oriented C4d on a surface appears to be dependent on a yet to be identified C4d receptor.-Hofer, J., Forster, F., Isenman, D. E., Wahrmann, M., Leitner, J., Hölzl, M. A., Kovarik, J. K., Stockinger, H., Böhmig, G. A., Steinberger, P., Zlabinger, G. J. Ig-like transcript 4 as a cellular receptor for soluble complement fragment C4d.

CD58/CD2 Is the Primary Costimulatory Pathway in Human CD28-CD8+ T Cells.

A substantial proportion of CD8(+) T cells in adults lack the expression of the CD28 molecule, and the aging of the immune system is associated with a steady expansion of this T cell subset. CD28(-)CD8(+) T cells are characterized by potent effector functions but impaired responses to antigenic challenge. CD28 acts as the primary T cell costimulatory receptor, but there are numerous additional receptors that can costimulate the activation of T cells. In this study, we have examined such alternative costimulatory pathways regarding their functional role in CD28(-)CD8(+) T cells. Our study showed that most costimulatory molecules have a low capacity to activate CD28-deficient T cells, whereas the engagement of the CD2 molecule by its ligand CD58 clearly costimulated proliferation, cytokine production, and effector function in this T cell subset. CD58 is broadly expressed on APCs including dendritic cells. Blocking CD58 mAb greatly reduced the response of human CD28(-)CD8(+) T cells to allogeneic dendritic cells, as well as to viral Ags. Our results clearly identify the CD58/CD2 axis as the primary costimulatory pathway for CD8 T cells that lack CD28. Moreover, we show that engagement of CD2 amplifies TCR signals in CD28(-)CD8(+) T cells, demonstrating that the CD2-CD58 interaction has a genuine costimulatory effect on this T cell subset. CD2 signals might promote the control of viral infection by CD28(-)CD8(+) T cells, but they might also contribute to the continuous expansion of CD28(-)CD8(+) T cells during chronic stimulation by persistent Ag.

Engagement of distinct epitopes on CD43 induces different co-stimulatory pathways in human T cells.

Co-receptors, being either co-stimulatory or co-inhibitory, play a pivotal role in T-cell immunity. Several studies have indicated that CD43, one of the abundant T-cell surface glycoproteins, acts not only as a potent co-receptor but also as a negative regulator for T-cell activation. Here we demonstrate that co-stimulation of human peripheral blood (PB) T cells through two distinct CD43 epitopes recognized by monoclonal antibodies (mAb) CD43-6E5 (T6E5-act ) and CD43-10G7 (T10G7-act ) potently induced T-cell proliferation. However, T-cell co-stimulation through two CD43 epitopes differentially regulated activation of nuclear factor of activated T cells (NFAT) and nuclear factor-κB (NF-κB) transcription factors, T-cell cytokine production and effector function. T6E5-act produced high levels of interleukin-22 (IL-22) and interferon-γ (IFN-γ) similar to T cells activated via CD28 (TCD28-act ), whereas T10G7-act produced low levels of inflammatory cytokines but higher levels of regulatory cytokines transforming growth factor-ß (TGF-ß) and interleukin-35 (IL-35). Compared with T6E5-act or to TCD28-act , T10G7-act performed poorly in response to re-stimulation and further acquired a T-cell suppressive function. T10G7-act did not directly inhibit proliferation of responder T cells, but formed stable heterotypic clusters with dendritic cells (DC) via CD2 to constrain activation of responder T cells. Together, our data demonstrate that CD43 is a unique and polarizing regulator of T-cell function.

Guanylate binding protein 1-mediated interaction of T cell antigen receptor signaling with the cytoskeleton.

GTPases act as important switches in many signaling events in cells. Although small and heterotrimeric G proteins are subjects of intensive studies, little is known about the large IFN-inducible GTPases. In this article, we show that the IFN-γ-inducible guanylate binding protein 1 (GBP-1) is a regulator of T cell activation. Silencing of GBP-1 leads to enhanced activation of early T cell Ag receptor/CD3 signaling molecules, including Lck, that is translated to higher IL-2 production. Mass spectrometry analyses showed that regulatory cytoskeletal proteins, like plastin-2 that bundles actin fibers and spectrin ß-chain, brain 1 that links the plasma membrane to the actin cytoskeleton, are binding partners of GBP-1. The spectrin cytoskeleton influences cell spreading and surface expression of TCR/CD3 and the leukocyte phosphatase CD45. We found higher cell spreading and enhanced surface expression of TCR/CD3 and CD45 in GBP-1 silenced T cells that explain their enhanced TCR/CD3 signaling. We conclude that GBP-1 is a downstream processor of IFN-γ via which T cells regulate cytoskeleton-dependent cell functions.

Resveratrol enhances TNF-α production in human monocytes upon bacterial stimulation.

BACKGROUND: Resveratrol is a key component of red wine that has been reported to have anti-carcinogenic and anti-aging properties. Additional studies conducted in vitro and in animal models suggested anti-inflammatory properties. However, data from primary human immune cells and in vivo studies are limited. METHODS: A pilot study was performed including 10 healthy volunteers. Plasma cytokine levels were measured over 48h after oral application of 5g resveratrol. To verify the in vivo findings, cytokine release and gene expression in human peripheral blood mononuclear cells (PBMC) and/or monocytes was assessed after treatment with resveratrol or its metabolites and stimulation with several toll-like receptor (TLR)-agonists. Additionally, the impact on intracellular signaling pathways was analyzed using a reporter cell line and Western blotting. RESULTS: Resveratrol treated individuals showed a significant increase in tumor necrosis factor-α (TNF-α) levels 24h after treatment compared to baseline. Studies using human PBMC or isolated monocytes confirmed potentiation of TNF-α production with different TLR agonists, while interleukin (IL)-10 was inhibited. Moreover, we observed significantly enhanced nuclear factor 'kappa-light-chain-enhancer' of activated B-cells (NF-κB) activation using a reporter cell line and found increased phosphorylation of p105, which is indicative of alternative NF-κB pathway activation. GENERAL SIGNIFICANCE: By administering resveratrol to healthy humans and utilizing primary immune cells we were able to detect TNF-α enhancing properties of the agent. In parallel, we found enhanced alternative NF-κB activation. We report on a novel pro-inflammatory property of resveratrol which has to be considered in concepts of its biologic activity.

p38α senses environmental stress to control innate immune responses via mechanistic target of rapamycin.

The MAPK p38α senses environmental stressors and orchestrates inflammatory and immunomodulatory reactions. However, the molecular mechanism how p38α controls immunomodulatory responses in myeloid cells remains elusive. We found that in monocytes and macrophages, p38α activated the mechanistic target of rapamycin (mTOR) pathway in vitro and in vivo. p38α signaling in myeloid immune cells promoted IL-10 but inhibited IL-12 expression via mTOR and blocked the differentiation of proinflammatory CD4(+) Th1 cells. Cellular stress induced p38α-mediated mTOR activation that was independent of PI3K but dependent on the MAPK-activated protein kinase 2 and on the inhibition of tuberous sclerosis 1 and 2, a negative regulatory complex of mTOR signaling. Remarkably, p38α and PI3K concurrently modulated mTOR to balance IL-12 and IL-10 expression. Our data link p38α to mTOR signaling in myeloid immune cells that is decisive for tuning the immune response in dependence on the environmental milieu.

ABO antibody and complement depletion by immunoadsorption combined with membrane filtration--a randomized, controlled, cross-over trial.

BACKGROUND: Potent antibody depletion techniques have paved the way to successful ABO-incompatible transplantation. Considering its efficiency regarding IgG removal, the use of non-antigen-specific semi-selective immunoadsorption (IA) has been advocated. One attractive strategy to overcome the caveat of incomplete IgM depletion and to interfere with complement activation could be the adjunctive use of membrane filtration (MF) to enhance the removal of macromolecules. METHODS: To investigate the depletion efficiency of semi-selective IA plus MF, we conducted a randomized, controlled, cross-over trial including patients on regular IA treatment for indications outside recipient desensitization. According to the results of sample size calculation, 14 subjects were enrolled. Two treatment sequences, a single session of IA plus MF followed by IA alone after ≥7 days (and vice versa), were analysed. RESULTS: IA plus MF markedly enhanced the median per cent reduction of ABO-specific IgM determined by flow cytometry (primary end point; 59 versus 23%, P < 0.001) and haemagglutination (2 versus 1 titre steps, P < 0.001), respectively. Combined treatment also substantially lowered C1q concentrations (86 versus 58% reduction, P < 0.001) and the functionality of classical complement as reflected by impaired in vitro C3 activation capability. IgG was strongly reduced without any additional effect of MF. CONCLUSIONS: We demonstrate that the innovative strategy of combining MF with semi-selective IA may substantially increase IgM elimination and affect classical complement activation. Our findings suggest that this new treatment concept could be an efficient strategy for recipient desensitization in ABO- and HLA-incompatible transplantation.