Efficacy of secukinumab and adalimumab in patients with psoriatic arthritis and concomitant moderate-to-severe plaque psoriasis: results from EXCEED, a randomized, double-blind head-to-head monotherapy study.

BACKGROUND: Secukinumab [an interleukin (IL)-17A inhibitor] has demonstrated significantly higher efficacy vs. etanercept (a tumour necrosis factor inhibitor) and ustekinumab (an IL-12/23 inhibitor) in patients with moderate-to-severe plaque psoriasis. OBJECTIVES: To report 52-week results from a prespecified analysis of patients with active psoriatic arthritis (PsA) having concomitant moderate-to-severe plaque psoriasis from the head-to-head EXCEED monotherapy study comparing secukinumab with adalimumab. METHODS: Patients were randomized to receive secukinumab 300 mg via subcutaneous injection at baseline, week 1-4, and then every 4 weeks until week 48 or adalimumab 40 mg via subcutaneous injection every 2 weeks from baseline until week 50. Assessments in patients with concomitant moderate-to-severe psoriasis, defined as having affected body surface area > 10% or Psoriasis Area and Severity Index (PASI) ≥ 10 at baseline, included musculoskeletal, skin and quality-of-life outcomes. Missing data were handled using multiple imputation. RESULTS: Of the 853 patients [secukinumab (N = 426), adalimumab (N = 427)], 211 (24·7%) had concomitant moderate-to-severe psoriasis [secukinumab (N = 110, 25·8%), adalimumab (N = 101, 23·7%)]. Up to week 50, 5·5% of patients discontinued secukinumab vs.17·8% in the adalimumab group. The proportion of patients who achieved American College of Rheumatology (ACR) 20 response was 76·4% with secukinumab vs. 68·3% with adalimumab (P = 0·175), PASI 100 response was 39·1% vs. 23·8% (P = 0·013), and simultaneous improvement in ACR 50 and PASI 100 response at week 52 was 28·2% vs. 17·7%, respectively (P = 0·06). Secukinumab demonstrated consistently higher responses vs. adalimumab across skin endpoints. CONCLUSIONS: This prespecified analysis in PsA patients with concomitant moderate-to-severe plaque psoriasis in the EXCEED study provides further evidence that IL-17 inhibitors offer a comprehensive biological treatment to manage the concomitant features of psoriasis and PsA.

Long-term safety of secukinumab in patients with moderate-to-severe plaque psoriasis, psoriatic arthritis, and ankylosing spondylitis: integrated pooled clinical trial and post-marketing surveillance data.

BACKGROUND: Secukinumab, a fully human immunoglobulin G1-kappa monoclonal antibody that directly inhibits interleukin (IL)-17A, has been shown to have robust efficacy in the treatment of moderate-to-severe psoriasis (PsO), psoriatic arthritis (PsA), and ankylosing spondylitis (AS) demonstrating a rapid onset of action and sustained long-term clinical responses with a consistently favorable safety profile in multiple Phase 2 and 3 trials. Here, we report longer-term pooled safety and tolerability data for secukinumab across three indications (up to 5 years of treatment in PsO and PsA; up to 4 years in AS). METHODS: The integrated clinical trial safety dataset included data pooled from 21 randomized controlled clinical trials of secukinumab 300 or 150 or 75 mg in PsO (14 Phase 3 trials and 1 Phase 4 trial), PsA (3 Phase 3 trials), and AS (3 Phase 3 trials), along with post-marketing safety surveillance data with a cut-off date of June 25, 2017. Adverse events (AEs) were reported as exposure-adjusted incident rates (EAIRs) per 100 patient-years. Analyses included all patients who received ≥ 1 dose of secukinumab. RESULTS: A total of 5181, 1380, and 794 patients from PsO, PsA, and AS clinical trials representing secukinumab exposures of 10,416.9, 3866.9, and 1943.1 patient-years, respectively, and post-marketing data from patients with a cumulative exposure to secukinumab of ~ 96,054 patient-years were included in the analysis. The most frequent AE was upper respiratory tract infection. EAIRs across PsO, PsA, and AS indications were generally low for serious infections (1.4, 1.9, and 1.2, respectively), Candida infections (2.2, 1.5, and 0.7, respectively), inflammatory bowel disease (0.01, 0.05, and 0.1, respectively), and major adverse cardiac events (0.3, 0.4, and 0.6, respectively). No cases of tuberculosis reactivation were reported. The incidence of treatment-emergent anti-drug antibodies was low with secukinumab across all studies, with no discernible loss of efficacy, unexpected alterations in pharmacokinetics, or association with immunogenicity-related AEs. CONCLUSIONS: Secukinumab demonstrated a favorable safety profile over long-term treatment in patients with PsO, PsA, and AS. This comprehensive assessment demonstrated that the safety profile of secukinumab was consistent with previous reports in patients with PsO, PsA, and AS, supporting its long-term use in these chronic conditions.

Secukinumab provides sustained PASDAS-defined remission in psoriatic arthritis and improves health-related quality of life in patients achieving remission: 2-year results from the phase III FUTURE 2 study.

BACKGROUND: Secukinumab has demonstrated sustained improvement in the signs and symptoms of psoriatic arthritis (PsA) over 2 years in the FUTURE 2 study (NCT01752634). This post hoc analysis assessed the ability of secukinumab to achieve Psoriatic Arthritis Disease Activity Score (PASDAS)-based remission or low disease activity (LDA) through 2 years among patients with PsA in the FUTURE 2 study. METHODS: PASDAS (cut-off scores: remission ≤ 1.9; LDA > 1.9 and < 3.2; Moderate Disease Activity ≥ 3.2 and < 5.4; and high disease activity [HDA] ≥ 5.4) was assessed in the overall population (tumour necrosis factor inhibitor [TNFi]-naïve and TNFi-experienced), in patients stratified by prior TNFi use and by disease duration at weeks 16, 52 and 104. The impact of secukinumab on individual PASDAS core components and on the relationship between PASDAS states and patient-reported outcomes (PROs), including physical function, health-related quality of life (HRQoL) and work productivity, were also assessed. Data for the approved doses of secukinumab (300 and 150 mg) are reported. PASDAS scores and core components were reported as observed, and PROs were analysed using mixed models for repeated measures. RESULTS: In the overall population, PASDAS remission and LDA were achieved in 15.6% and 22.9%, respectively, of patients treated with secukinumab 300 mg and in 15.2% and 19.2%, respectively, in the secukinumab 150 mg group versus 2.3% and 13.8%, respectively, with placebo at week 16. In the TNFi-naïve group, a higher proportion of patients achieved remission + LDA at week 16 with secukinumab 300 and 150 mg (46.2% and 42.9%, respectively) versus placebo (17.5%), with corresponding responses in TNFi-experienced patients being 22.6% and 19.4% versus 13.3%. Remission/LDA responses with secukinumab were sustained through 2 years. Patients achieving remission/LDA reported greater improvements in PROs than patients in HDA through 2 years. CONCLUSIONS: Secukinumab-treated patients achieved higher PASDAS-defined remissions or LDA compared with placebo at week 16, which were sustained through 2 years. Remission/LDA was achieved by both TNFi-naïve and TNFi-experienced patients treated with secukinumab, with higher rates in TNFi-naïve patients. Secukinumab-treated patients achieving remission/LDA reported significantly greater improvements in PROs, including physical function and different dimensions of health-related quality of life and work, than patients in HDA. TRIAL REGISTRATION: ClinicalTrials.gov, NCT01752634 . Registered on December 19, 2012. EUDRACT, 2012-004439-22 . Registered on December 12, 2012.

Characterization of the FcgammaRIIA promoter and 5'UTR sequences in patients with systemic lupus erythematosus.

FcgammaRIIA is a candidate gene involved in the predisposition to systemic lupus erythematosus (SLE). The presence of low binding alleles in patients with SLE is not sufficient to explain the lower phagocytic capacity observed in SLE patients. We considered the possibility that nucleotide polymorphisms in the FcgammaRIIA promoter that cause alterations in receptor expression might be present in SLE patients. In the present study, a 2.0 kb region of the human FcgammaRIIA 5'UTR from 20 normal donors and 53 SLE patients was examined. The results demonstrate that the sequence of the human FcgammaRIIA 5' region differs from the published sequence. Two novel SNPs have been identified in the distal region of the FcgammaRIIA promoter. The polymorphisms are present in both disease-free and SLE donors and do not associate with quantitative changes in FcgammaRIIa phagocytic function.

Human receptors for immunoglobulin G: key elements in the pathogenesis of rheumatic disease.

The structural diversity of Fc gammaR provides a mechanism by which IgG can elicit a broad range of cell responses. Fc gammaR vary in their affinity for IgG, their preference for IgG subclasses, the cell types in which they are expressed, and the intracellular signals which they elicit--stimulatory or inhibitory. Expansion in our knowledge of structure-function relationships among Fc gammaR has identified them as heritable risk factors for disease susceptibility and valuable targets for therapeutic modulation of the immune system.

Differential modulation of stimulatory and inhibitory Fc gamma receptors on human monocytes by Th1 and Th2 cytokines.

Immune complex-mediated inflammatory responses are initiated by Fc gamma R on phagocytes. We report in this study that an inhibitory receptor, Fc gamma RIIb2, is expressed on circulating human monocytes, and when co-cross-linked with stimulatory Fc gamma R it down-regulates effector function. Fc gamma RIIb2 expression is increased by IL-4 and decreased by IFN-gamma, in contrast to the activating receptor, Fc gamma RIIa, which is increased by IFN-gamma and decreased by IL-4. Thus, Th1 and Th2 cytokines differentially regulate the opposing Fc gamma R systems, altering the balance of activating and inhibiting Fc gamma R. The detection and cytokine modulation of Fc gamma RIIb2 in human myeloid cells provide evidence of a negative regulator of immune complex-mediated responses in human phagocytes and offer a new approach to limit Ab-triggered inflammation in autoimmune disease.

Reactive oxygen intermediates enhance Fc gamma receptor signaling and amplify phagocytic capacity.

Receptors for the Fc region of IgG (Fc gamma R) mediate internalization of opsonized particles by human neutrophils (PMN) and mononuclear phagocytes. Cross-linking of Fc gamma R leads to activation of protein tyrosine kinases and phosphorylation of immunoreceptor tyrosine-based activation motifs (ITAMs) within Fc gamma R subunits, both obligatory early signals for phagocytosis. Human PMN constitutively express two structurally distinct Fc gamma R, Fc gamma RIIa and Fc gamma RIIIb, and can be induced to express Fc gamma RI by IFN-gamma. We have previously shown that stimulation of PMN through Fc gamma RIIIb results in enhanced Fc gamma RIIa-mediated phagocytic activity that is inhibited by catalase. In the present study, we have tested the hypothesis that reactive oxygen intermediates (ROI) have the capacity to regulate Fc gamma R responses and defined a mechanism for this effect. We show that H2O2 augmented phagocytosis mediated by Fc gamma RIIa and Fc gamma RI in PMN and amplified receptor-triggered tyrosine phosphorylation of Fc gamma R-associated ITAMs and signaling elements. Generation of endogenous oxidants in PMN by cross-linking Fc gamma RIIIb similarly enhanced phosphorylation of Fc gamma RIIa and Syk, a tyrosine kinase required for phagocytic function, in a catalase-sensitive manner. Our results provide a mechanism for priming phagocytes for enhanced responses to receptor-driven effects. ROI generated in an inflammatory milieu may stimulate quiescent cells to rapidly increase the magnitude of their effector function. Indeed, human monocytes incubated in the presence of stimulated PMN showed oxidant-induced increases in Fc gamma RIIa-mediated phagocytosis. Definition of the role of oxidants as amplifiers of Fc gamma R signaling identifies a target for therapeutic intervention in immune complex-mediated tissue injury.

A naturally occurring mutation in Fc gamma RIIA: a Q to K127 change confers unique IgG binding properties to the R131 allelic form of the receptor.

Fc gamma RIIa is widely expressed on hematopoietic cells. There are two known allelic polymorphic forms of Fc gamma RIIa, Fc gamma RIIa-R131 and Fc gamma RIIa-H131, which differ in the amino acid at position 131 in the second lg-like domain. In contrast to Fc gamma RIIa-R131, Fc gamma RIIa-H131 binds hlgG2 but not mIgG1, and this differential binding has clinical implications for host defense, autoimmune disease, immunohematologic disease, and response to therapeutic monoclonal antibodies. We identified a novel Fc gamma RIIA genotype in a healthy individual homozygous for Fc gamma RIIA R/R131 in whom a C to A substitution at codon 127 changes glutamine (Q) to lysine (K) in one of the two Fc gamma RIIA genes. This individual's homozygosity for Fc gamma RIIA-R/R131 leads to the prediction that the receptors on her cells would not bind hIgG2. Monocyte and neutrophil phagocytosis of hIgG2-opsonized erythrocytes was significantly higher (P < .05) for cells from this K/Q127, R/R131 individual than for Q/Q127, R/R131 donors. Platelet aggregation stimulated by an mIgG1 anti-CD9 antibody in this individual was significantly different (P < .05) from Q/Q127, H/R131 and Q/Q127, H/H131 donors and similar to Q/Q127, R/R131. Our data show that the K127/R131 receptors have a unique phenotype, binding both hIgG2 and mIgG1. Further functionally significant mutations in human Fc gamma receptors and possible novel mechanisms for inherited differences in disease susceptibility should be sought with unbiased screening methods.

Flow cytometric quantitation of attachment and phagocytosis in phenotypically-defined subpopulations of cells using PKH26-labeled Fc gamma R-specific probes.

Human receptors for IgG (Fc gamma R) are characterized by diverse structure and function. We describe a flow cytometric technique to quantitate receptor-specific Fc gamma R-mediated attachment and phagocytosis in phenotypically-defined subsets of cells using erythrocytes (E) labeled with the lipophilic dye PKH26 and coupled with biotin/avidin to either human IgG (myeloma proteins) or anti-Fc gamma R mAb. Using this technique and Fc gamma RIIa as a model, (1) we demonstrate sensitive and specific quantitation of attached and internalized E coupled to receptor-specific mAb or natural ligand by monocytes within a peripheral blood leukocyte preparation; (2) we show the capacity to detect subtle allelic differences in Fc gamma R function; and (3) we demonstrate oxidant-induced enhancement of binding and internalization.

V lambda-light chain genes reconstitute immune responses to defined carbohydrate antigens or haptens by utilizing different VH genes.

The contribution of the lambda-light chain to the development of peripheral B cell repertoire and generation of specific antibodies to haptens and polysaccharide antigens was studied in genetically manipulated kappa-deficient and lambda 2-transgenic mice. The results clearly demonstrate a non-stoichiometric VH gene family expression in the absence of k-light chain and suggest a non-stochastic pairing between VH and V lambda genes, expressed in the peripheral B cell repertoire. A shift in VH gene utilization in the case of VI lambda + antibodies was evident in response to beta 2-6 fructosan and TNP hapten. These observations demonstrate the availability of compensatory mechanisms in the absence of VK genes and are consistent with the hypothesis that VH gene family expression is controlled by genetic factors from inside the VH locus. Furthermore, genetic factors from outside the VH locus, namely restricted available light chain diversity, may lead to a shift in VH gene utilization in the peripheral B cell repertoire.

Repertoires of antigen receptors in Tdt congenitally deficient mice.

Tdt deficient mice show lack of N region in V(D)J junctions of immunoglobulin and T cell receptor genes and revealed that "immature recombinase" in fetal stage would boil down to no more than a lack of Tdt. Although particular junctions which are thought to be created by homology-mediated joining are frequently observed, one fourth of junctions lacked even one bp of overlap, indicating the existence of a V(D)J joining pathway that is homology independent. Lymphocyte repertoire which express VH81X gene without N region is negatively selected, which shows that the repertoire of Tdt deficient mice is not a truly fetal repertoire. Positive selection of thymocytes is more efficient in Tdt deficient mice. Furthermore Tdt-/- mice produce significant amounts of anti-dsDNA antibodies as Tdt+/+ mice, indicating that increased diversity of the third complementarity-determining region (CDR3) by Tdt is not essential for the expansion of precursor B cells programmed to produce anti-DNA antibodies.

Antibody response against poly (Glu60Ala30Tyr10) terpolymer and bacterial levan in kappa-deficient mice.

In murine species, the kappa (+)-bearing immunoglobulins dominate the antibody (Ab) repertoire with a kappa/lambda ratio of 95:5. The aim of the present study is to investigate the characteristics of the antibody response in kappa-deficient (K-/-) mice immunized with a T-dependent synthetic antigen, poly(Glu60Ala30Tyr10) (GAT) and a T-independent antigen, bacterial levan (BL). K-/- mice were obtained by targeted deletion of the J kappa C kappa gene segments. In response to GAT, K-/- mice respond by producing increasing amounts of anti-GAT Ig lambda 1 and Ig lambda 2 in the primary as well as secondary response, although anti-GAT specific monoclonal antibodies (mAb) raised in K-/- mice are mostly of IgM isotype. The GAT public idiotype, GATIdX, present on all GAT-specific Ab bearing kappa light chain, is not detected in the sera of K-/- mice or on any of the anti-GAT lambda 1 mAb. In response to BL, the amount of Ig lambda 1+ Ab in K-/- mice is comparable to the amount of Ig kappa + Ab in normal mice. However, lambda 2+ Ab are detected neither in wild-type nor in K-/- mice. Like kappa + Ab, the majority of lambda 1+ mAb are specific for beta 2-6 fructosan present in BL and rye levan and, to some extent, express the BL-specific idiotype, A48ld. Our results show that important compensatory mechanisms occur in kappa-deficient mice, restoring their ability to mount immune responses against a variety of T-dependent and T-independent antigens by the alternative usage of the clonally restricted lambda repertoire.

Study of self reactive antibodies in kappa-light chain deficient mice.

The kappa chain deficient mouse strain represents an excellent experimental system for studying the contribution of lambda light chains to the antibody repertoire. Here, we have studied the contribution of lambda chains to the generation of self reactive antibodies including RFs in kappa deficient mice with 129/sv background. These mutant mice produce rheumatoid factors similar to 129/sv mice and these antibodies are primarily encoded by lambda 2. This may be due to the production of RF by peritoneal B lymphocytes which belong to Ly1 B subset. Peritoneal B cell selectively produce lambda 2 and lambda 3 isotypes. Though lambda 1 positive RF is not detectable in the sera, lambda 1 positive specific precursor B cells are present in these mice and they can be activated by T-independent antigens. Our studies show that these mice also spontaneously produce anti-dsDNA antibodies bearing lambda 1 light chain but do not produce self reactive antibodies specific for eight different autoantigens. However, B cell precursors expressing lambda chains specific for autoantigens like collagen II, III, IV and histone 2A are present in the B cell repertoire of kappa-deficient mice. Thus, our results demonstrate that lambda light chain can compensate, to some extent, the lack of kappa chain repertoire, not only against foreign antigens, as observed previously, but also against a number of autoantigens.

Antibody response elicited by T-dependent and T-independent antigens in gene targeted kappa-deficient mice.

Animal models substantially contribute to the understanding of the pathogenesis of various human diseases, including those associated with genetic defects. Our study investigated the characteristics of antibody responses elicited by T-dependent and T-independent antigens in mice rendered kappa-deficient by targeted deletion of the J kappa C kappa gene segments. It is known that in normal murine species the kappa repertoire dominates the antibody repertoire (kappa/lambda ratio = 95:5). Our results indicate that the kappa gene deletion causes the alternative usage of lambda 1 (93%) and lambda 2 (7%) light chains, confirming previous studies demonstrating that in kappa-deficient mice all B cells express Ig lambda receptors. The anti-trinitrophenylbenzene (TNP) response in K-/- mice was compensated for by lambda 1 and lambda 2 bearing Igs. However, isoelectric focusing analysis of anti-TNP antibodies showed a considerably more restricted pattern of lambda anti-TNP antibodies in K-/- as compared with kappa antibodies in normal mice. No major differences were observed in the affinity for the hapten of kappa or lambda 1 or lambda 2 mAbs obtained from 129/Sv and K-/- mice. Furthermore, lambda 1 and lambda 2 chains can reconstitute the expression of an idiotype (460Id) borne on kappa anti-TNP antibodies. The 460Id was detected both in polyclonal and monoclonal anti-TNP antibodies obtained from K-/- mice. Our results clearly showed that the kappa anti-TNP repertoire is compensated by the lambda repertoire even though the latter is clonally restricted in K-/- mice.

Expression and function of CD7 molecule on human natural killer cells.

The CD7 molecule, one of the earliest T-lymphocyte Ag expressed during ontogeny, has recently been demonstrated to facilitate activation of T cells and to preferentially activate TCR-gamma/delta + subset of T cells. The CD7 Ag is also expressed on human NK cells, but its function has not been determined. In this study, expression and function of CD7 Ag on highly enriched NK cells (94 +/- 3% mean +/- SD, n = 12) obtained by negative selection from peripheral blood of normal donors were investigated. The CD7 Ag was found to be expressed at a significantly (p < 0.002) higher level on fresh NK cells than on IL-2-activated, NK cells. CD7 on human NK cells was found to be a signal-transducing molecule with a rapid increase in cytoplasmic free calcium observed on binding of anti-CD7 mAb to the surface of NK cells. Cross-linking of CD7 induced expression of surface activation molecules such as CD25, CD71, HLA-DR, CD69, and CD54. Activation by anti-CD7 mAb cross-linked to plastic or through goat anti-mouse Ig also induced a variety of NK cell functions: it stimulated secretion of IFN-gamma, led to proliferation of NK cells, as measured by [3H]thymidine incorporation, and significantly enhanced cytotoxicity of NK cells against K562 targets (p < 0.03). However, CD7 on NK cells did not seem to transduce a lytic signal, because it neither mediated redirected killing of Fc gamma R+ murine mastocytoma P815 cells nor triggered lysis of a hybridoma expressing the antibody in a membrane-bound form. CD7 molecules appeared to have a regulatory role in adhesion of NK cells to fibronectin, because cross-linking of CD7 on resting NK cells significantly augmented their adhesion to fibronectin-coated plastic surfaces. However, this induced adhesion was not associated with increased expression of beta 1-integrins on NK cells. Thus, CD7-mediated signals appear to augment function of adhesion molecules on NK cells, which may be involved in NK cell activation by providing both anchorage and costimulatory triggering.

Characterization of the Fc mu receptor on human natural killer cells. Interaction with its physiologic ligand, human normal IgM, specificity of binding, and functional effects.

After treatment with human normal IgM, 78 +/- 8% of purified CD3-CD56+ resting human NK cells and 93 +/- 6% of IL-2-activated NK cells selected by adherence to plastic reacted with FITC-goat anti-human IgM. Binding of IgM to the FcR for IgM (Fc mu R) on human NK cells was not species specific because mouse myeloma IgM also bound to these cells. The percentage of CD56+ cells binding IgM after incubation with anti-CD16 mAb was similar to that of cells incubated with medium alone (95 +/- 1% vs 93 +/- 4%). Binding of anti-CD16 mAb to Fc gamma RIII on NK cells was unaffected by pretreatment with IgM (65 +/- 12% vs 69 +/- 4%). The CD7 molecule has been reported to be the Fc mu R on the surface of T cells. Two-color flow cytometry showed that 94 +/- 3% of CD3-CD56+ resting NK cells and 71 +/- 16% of activated NK cells were CD7+. Preincubation of NK cells with three anti-CD7 mAb (Leu-9, 8H8-1, and LAU-A1) failed to block the binding of IgM to the Fc mu R. Modulation of the CD7 molecule off the cell surface (CD7+ = 1.5% +/- 0.3) did not reduce IgM binding, thus excluding the possibility that IgM anti-CD7 might bind to different epitopes of the same molecule. These data indicate that the Fc mu R is a specific Ig-binding structure, distinct from the Fc gamma RIII (CD16) or CD7. The Fc mu R on NK cells functions as a signal-transducing molecule because the addition of 0.2 mg/ml IgM to R-NK cells caused a rapid increase in [Ca2+]i (delta = 40 nM). One of the early events that followed signaling through the Fc mu R was the down-modulation of IFN-gamma gene expression and IFN gamma production in NK cells. The presence of IgM during culture of NK cells consistently decreased the expression of HLA-DR (16% vs 40% in control). Thus, the Fc mu R, a constitutively-expressed receptor on human NK cells, seems to be an important functional molecule, which delivers negative regulatory signals to NK cells.

Expression of Fc mu receptors on human natural killer cells.

Fc receptors for IgG (CD16) have been described as the only type of immunoglobulin receptor on large granular lymphocytes (LGL). However, the ability of natural killer (NK) cells to mediate antibody-dependent cellular cytotoxicity (ADCC) in the presence of monoclonal or polyclonal IgM and the inhibition of NK activity by highly purified IgM could not be explained on the basis of FcR for IgG. In order to directly assess the expression of Fc receptors for IgM (Fc mu R), NK cells were treated with human polyclonal IgM, and its binding was visualized by a direct anti-globulin rosette assay with identification of rosette-forming LGL on Giemsa-stained smears. The data indicated that a high proportion of LGL (up to 68%) were Fc mu R-positive cells. However, this percentage varied depending on the IgM preparation (polyclonal or monoclonal), the indicator reagent used for the rosette assays, and the cell preparations studied. Two-color flow cytometry of human nonadherent lymphocyte preparations confirmed the presence of CD56+IgM+ cells, which represented from 43 to 78% of CD56+ cells. Flow cytometry was also performed using highly enriched preparations of human NK cells (the mean percentage of CD3-CD56+ cells was 84%). Up to 88% of purified NK cells bound FITC-labeled monoclonal IgM at a saturating concentration. By indirect immunofluorescence, from 34 to 62% of NK cells purified from the peripheral blood of normal donors were able to bind polyclonal IgM. Similar results were obtained with LGL from a patient with NK lymphoproliferative disease. Thus the presence of Fc mu R on a majority of human NK cells was demonstrated by different techniques, using unseparated peripheral blood mononuclear leukocytes, purified normal NK cells, and also LGL from a patient with NK lymphoproliferative disease.

Inhibition of human natural killer cell activity by polyclonal IgM.

Pretreatment of effector cells with normal human IgM induced strong dose-dependent inhibition of NK activity. The degree of inhibition by normal IgM was stronger than that induced by monomeric IgG, which has previously been reported to be a negative regulator of NK activity. For 100% inhibition, 1.1 x 10(-6) M of IgM was required, whereas 66.6 x 10(-6) M of IgG was needed to abolish NK activity. This inhibitory property of polyclonal IgM appeared to be localized in the Fc region of the molecule, and also was significantly reduced upon mild reduction of disulfide bonds. Monoclonal IgM purified from sera of five patients with Waldenström's macroglobulinemia and tested in parallel with normal IgM lacked or had a decreased capacity to inhibit the cytotoxic reaction. As with IgG, IgM interfered mainly with the lytic event, after binding of effector cells to target cells. The inhibition by IgM appeared to be a direct effect on NK cells, since similar effects were observed with purified large granular lymphocytes as with non-adherent lymphocytes. These results indicate a new mechanism for negative regulation of NK cells and suggest the presence of Fcmu receptors on these effector cells.