Role Of Antibody-dependent Cellular Cytotoxicity Research Articles

E‐cadherin expression on human carcinoma cell affects trastuzumab‐mediated antibody‐dependent cellular cytotoxicity through killer cell lectin‐like receptor G1 on natural killer cells

Trastuzumab is a recombinant antibody drug that is widely used for the treatment of HER2-overexpressing breast carcinoma. Despite encouraging clinical results, many HER2-overexpressing carcinomas are primarily resistant to trastuzumab. We attempted to explain trastuzumab resistance and search for solutions. Since the killer cell lectin-like receptor G1 (KLRG1), an inhibitory receptor expressed on subsets of natural killer (NK) cells recognizes E-cadherin as ligands and may inhibit immune responses by regulating the effector function of NK cells, we used HER2-overexpressing carcinoma cells which were expressing E-cadherin to investigate the role of antibody-dependent cellular cytotoxicity (ADCC) through KLRG1 on NK cells in vitro and vivo. The results indicated that HER2-overexpressing carcinoma cells were killed by trastuzumab-mediated ADCC and the ADCC activity was reflected the degree of E-cadherin expression on carcinoma cells. We found that expression of E-cadherin was shown to be a predictor of response to trastuzumab-based treatment for HER2-overexpressing carcinomas, furthermore, trastuzumab-mediated ADCC was markedly enhanced by KLRG1-negative peripheral blood mononuclear cells (PBMCs(KLRG1(-))).

Antibody-Dependent Natural Killer Cell–Mediated Cytotoxicity Engendered by a Kinase-Inactive Human HER2 Adenovirus-Based Vaccination Mediates Resistance to Breast Tumors

Cancer vaccines may have applications in the therapy and prevention of mammary carcinoma. To investigate such applications, we constructed a recombinant adenoviral vaccine expressing a kinase-inactive mutant form of human HER2 and introduced this into BALB/c wild-type (WT) or HER2 transgenic mice. Here, we report contributions by antibody responses and natural killer (NK) cells in tumor protection in this model. One i.p. vaccination protected WT mice from the HER2-expressing mouse carcinoma D2F2/E2. Half of the HER2 transgenic mice were protected fully and long term after preventive vaccination. Tumor growth in mice that eventually developed neoplastic lesions was delayed. Protection in WT and HER2 transgenic mice was associated with high or low levels of IgG2a antibodies, respectively, whereas CTLs were observed in WT but not in HER2 transgenic mice. Depleting CD4(+) or CD8(+) cells in vaccinated WT mice had limited effects, suggesting that protection was largely independent of CD4(+) or CD8(+) T cells. In contrast, antibody-mediated tumor rejection seemed to contribute significantly based on a loss of protection in mice deficient for Fc-γ RI/III or B cells. Further, a role for antibody-dependent cellular cytotoxicity (ADCC) mediated by NK cells was indicated by evidence that vaccine protection could be abolished by in vivo depletion of NK cells. Lastly, NK cells and immune sera purified from WT or HER2 transgenic mice exhibited efficient ADCC of HER2-expressing tumor cells in vitro. Our findings define a critical requirement for NK cells in vaccine-induced protection against HER2-expressing tumors.

A dimeric form of the HIV-1 antibody 2G12 elicits potent antibody-dependent cellular cytotoxicity.

Increasing data support a role for antibody-dependent cellular cytotoxicity (ADCC) in controlling HIV-1 infection. We recently isolated a naturally occurring dimeric form of the anti-HIV-1 antibody 2G12 and found it to be significantly more potent than 2G12 monomer in neutralizing primary virus strains. However, given the unusual structure of dimeric 2G12 with two Fc regions, it was not clear whether 2G12 dimer could bind to the CD16 Fc receptor on ADCC effector cells or trigger ADCC. Here we compared the in-vitro ADCC activities of 2G12 monomer and dimer and investigated the effects of including ADCC-enhancing mutations in both forms of 2G12. An in-vitro ADCC assay using target cells stably expressing gp160 was developed to evaluate the activities of 2G12 monomer and dimer with and without ADCC-enhancing mutations that increase the CD16-binding affinity of the 2G12 Fc region. Both 2G12 monomer and 2G12 dimer elicited ADCC, although the dimer showed increased potency [lower half-maximal concentration (EC(50))] in triggering ADCC, thus confirming its ability to bind CD16 and trigger ADCC. The ADCC-enhancing mutations improved the ADCC activity of 2G12 monomer more than 2G12 dimer such that their EC(50) values were nearly equal. However, no increase in nonspecific ADCC activity was observed using 2G12 IgGs with these mutations. Given the likelihood that ADCC plays a role in protecting against initial infection and/or controlling chronic infection, these data suggest 2G12 dimers and/or addition of ADCC-enhancing mutations could augment the prophylactic and/or therapeutic potential of 2G12.

Immunoglobulin G Fc Receptor FcγRIIIa 158 V/F Polymorphism Correlates With Rituximab-Induced Neutropenia After Autologous Transplantation in Patients With Non-Hodgkin's Lymphoma

Rituximab has been given after autologous hematopoietic cell transplantation for recurrent or refractory B-cell lymphoma with the goal of eradicating minimal residual disease. Our previous report showed that administration of two courses of rituximab after transplantation is feasible, with encouraging clinical outcomes after a short follow-up. However, neutropenia after the first or second post-transplantation rituximab treatment occurred in 52% of patients. We previously reported that polymorphisms of two immunoglobulin G Fc receptors predict rituximab response, presumably because of their role in antibody-dependent cellular cytotoxicity. In the current report, we determine whether FcgammaR polymorphisms are correlated with clinical outcomes in 33 patients with B-cell non-Hodgkin's lymphoma who received post-transplantation rituximab. Genomic DNA was used for FcgammaRIIIa V/F or the FcgammaRIIa H/R genotyping. The FcgammaR polymorphisms were then correlated with the incidence of rituximab-induced neutropenia, event-free survival (EFS), and overall survival (OS). The FcgammaRIIIa 158 V allele dose was correlated with a higher incidence of rituximab-induced neutropenia. The odds of neutropenia after the first or second post-transplantation rituximab increased three-fold with each V allele (robust z = 2.08, P = .038). The FcgammaRIIa polymorphism had no impact on rituximab-induced neutropenia. We did not observe a correlation of either FcgammaRIIIa or FcgammaRIIa polymorphism with EFS or OS. The high affinity FcgammaRIIIa 158 V allele is associated with rituximab-induced neutropenia after autologous transplantation. This is a potential tool to identify a high-risk population for developing neutropenia after antibody therapy.

Histopathological analyses of the antitumor activity of anti-glypican-3 antibody (GC33) in human liver cancer xenograft models: The essential role of macrophages

Previously, we demonstrated the antitumor efficacy of the anti-glypican-3 (GPC3) antibody GC33 in several human liver cancer xenograft models and the important role of antibody-dependent cellular cytotoxicity (ADCC) in the antitumor mechanism of GC33. Involvement of other mechanisms such as modulation of the functions of GPC3 in antitumor activity remains to be elucidated. In this study, we investigated histopathologically time-course changes in xenografts in mice following a single administration of GC33 to clarify the morphological changes contributing to the tumor growth inhibition of GC33, including the changes in GPC3-related factors/components (proliferation, extracelluar matrices (ECMs) and macrophage). Histopathological changes peaked 3–5 days after GC33 administration and included increased tumor cell death, tumor cells with round morphology, multinucleated tumor cells and small spindle/round-like cells (mostly F4/80-positive macrophages). No direct effects of GC33 on proliferation activity of tumor cells were observed. Meanwhile, alteration of ECM structures and a remarkable increase in macrophages was noted in the GC33-treated group. Increase in macrophages was observed mainly in the outer layer of tumor nodules; the area of the increase approximately included the area where the change in tumor cells and ECMs were observed. Interestingly, depletion of macrophages in the xenograft models resulted in a marked reduction of the antitumor activity of GC33. In the in vitro ADCC assay, ADCC was only slightly induced by mouse peritoneal macrophages. These data suggest that macrophages play an essential role in the antitumor activity of GC33 and the possible involvement of macrophage-mediated non-ADCC action.

FcgammaRIIIa polymorphisms and cetuximab induced cytotoxicity in squamous cell carcinoma of the head and neck.

The interaction of Fc fragments of antibodies with the Fcgamma receptors is an essential checkpoint in antibody-dependent cellular cytotoxicity (ADCC). Specific polymorphisms at position 158 enhance FcgammaRIIIa affinity for IgG1 and are associated with improved clinical outcome in lymphoma patients treated with IgG1 anti-CD20 antibody. The role of ADCC in the therapeutic effects of the alpha-epidermal growth factor receptor (EGFR) mAb, cetuximab, in patients with squamous cell carcinoma of the head and neck (SCCHN) is poorly defined. We employed three SCCHN cell lines to test two hypotheses: (1) SCCHN is susceptible to cetuximab-mediated ADCC, (2) efficacy of ADCC is associated with polymorphisms at position 158 of FcgammaRIIIa. FcgammaRIIIa-158 polymorphisms were determined for healthy donors, and their purified NK cells were used as effector cells against three SCCHN cell lines in ADCC assays. Cytotoxicity levels were compared for each polymorphism class. Proliferation and cell cycle assays were done to examine the direct effects of cetuximab. Our results indicate that SCCHN is susceptible to cetuximab-mediated ADCC in vitro. NK cytotoxic efficiency correlates with donor 158-polymorphisms in FcgammaRIIIa. Overall cytotoxicity was greatest for individuals having a single V allele when compared to homozygous F/F individuals; the cumulative percent cytotoxicity for each polymorphism among the cell lines was 58.2% V/V, 50.6% V/F, and 26.1% F/F (P < 0.001). Additionally, the presence of a V allele correlated with superior natural cytotoxicity against NK sensitive targets. These data have both prognostic and therapeutic relevance and support the design of a prospective trial to determine the influence of FcgammaRIIIa polymorphisms on the clinical outcome of patients with SCCHN treated with alpha-EGFR mAbs.

Trastuzumab causes antibody-dependent cellular cytotoxicity–mediated growth inhibition of submacroscopic JIMT-1 breast cancer xenografts despite intrinsic drug resistance

Trastuzumab is a recombinant antibody drug that is widely used for the treatment of breast cancer. Despite encouraging clinical results, some cancers are primarily resistant to trastuzumab, and a majority of those initially responding become resistant during prolonged treatment. The mechanisms of trastuzumab resistance have not been fully understood. We examined the role of antibody-dependent cellular cytotoxicity (ADCC) using JIMT-1 cells that are ErbB2 positive but intrinsically resistant to trastuzumab in vitro. Unexpectedly, in experiments mimicking adjuvant therapy of submacroscopic disease in vivo (JIMT-1 cells inoculated s.c. in severe combined immunodeficiency mice), trastuzumab was able to inhibit the outgrowth of macroscopically detectable xenograft tumors for up to 5-7 weeks. The effect is likely to be mediated via ADCC because trastuzumab-F(ab')(2) was ineffective in this model. Moreover, in vitro ADCC reaction of human leukocytes was equally strong against breast cancer cells intrinsically sensitive (SKBR-3) or resistant (JIMT-1) to trastuzumab or even against a subline of JIMT-1 that was established from xenograft tumors growing despite trastuzumab treatment. These results suggest that ADCC may be the predominant mechanism of trastuzumab action on submacroscopic tumor spread. Thus, measuring the ADCC activity of patient's leukocytes against the tumor cells may be a relevant predictor of clinical trastuzumab responsiveness in vivo.

The anti‐CD20 antibody rituximab augments the immunospecific therapeutic effectiveness of an anti‐CD19 immunotoxin directed against human B‐cell lymphoma

The chimaeric anti-CD20 antibody rituximab (Rituxan) sensitises lymphoma cells to small molecule cytotoxic drugs and to protein toxins. We have explored the augmentive effect of rituximab on the anti-CD19 immunotoxin BU12-SAPORIN in a model of human lymphoma. Intact rituximab and its F(ab)2 derivative both augmented the immunospecific protein synthesis inhibitory effects of BU12-SAPORIN in a complement-independent manner. A combination of rituximab + BU12-SAPORIN completely abolished the proliferation of Ramos cells in vitro and also induced a significantly greater degree of apoptosis in these cells. Treatment with rituximab, BU12-SAPORIN or a combination of both induced poly(ADPribose) polymerase and caspase 3 cleavage, although this was always consistently greater in combination-treated cells. zVAD almost completely inhibited apoptosis in rituximab- or BU12-SAPORIN-treated cells but only partially in combination-treated cells. In severe combined immunodeficient (SCID)-Ramos mice the combination of rituximab + BU12-SAPORIN was significantly better therapeutically than either single agent. The immunological fidelity of the therapeutic effect because of combination treatment was demonstrated through the failure of rituximab to augment an irrelevant anti-CD7 immunotoxin. The therapeutic efficacy of rituximab and combination treatment was reduced in SCID-Ramos mice depleted of serum complement while natural killer cell depletion failed to show any convincing role for antibody-dependent cellular cytotoxicity. This study shows a clear therapeutic advantage from using rituximab to immunospecifically augment immunotoxin cytotoxicity warranting further investigation.

Alemtuzumab induces enhanced apoptosis in vitro in B-cells from patients with chronic lymphocytic leukemia by antibody-dependent cellular cytotoxicity

Alemtuzumab, a monoclonal anti-CD52 antibody has been shown to be highly effective in B-cell chronic lymphocytic leukemia, even in fludarabine-refractory disease. The mechanism of action is currently unknown, but may rely on complement-mediated cell lysis and antibody-dependent cellular cytotoxicity. The aim of this study was to assess the proapoptotic activity of alemtuzumab in chronic lymphocytic leukemia and to describe pathways potentially underlying this effect. Peripheral blood mononuclear cells from 21 chronic lymphocytic leukemia patients were treated in vitro in the absence of complement with fludarabine alone, alemtuzumab alone, or with the additional presence of a cross-linking anti-Fc-antibody. Apoptosis was quantified after 24 h by flow cytometry analysis. Expression of several pro- and anti-apoptotic proteins was determined at different time points. Apoptosis of peripheral blood mononuclear cells treated with alemtuzumab alone was significantly enhanced compared to untreated cells suggesting a minor potentially cytotoxic mechanism by direct signaling independent from antibody-dependent cellular cytotoxicity. The presence of a cross-linking anti-Fc-antibody induced the formation of cell clusters and enhanced apoptosis significantly suggesting a potential role of antibody-dependent cellular cytotoxicity in alemtuzumab induced apoptosis. Alemtuzumab activated a CD52-dependent signaling pathway which induced a significant increase in caspase 3 and 8 expression. Alemtuzumab significantly enhances apoptosis in chronic lymphocytic leukemia cells in vitro, especially in combination with a cross-linking anti-Fc-antibody, this effect being mediated by a caspase-dependent pathway.

Relationship between antibody-dependent cellular cytotoxicity, plasma HIV type 1 RNA, and CD4+ lymphocyte count.

To explore the role of antibody-dependent cellular cytotoxicity (ADCC) in controlling HIV-1 infection, ADCC was compared with plasma RNA and CD4+ cell count in 40 patients not receiving antiretroviral therapy and in seven patients after the initiation of treatment. Among untreated patients, ADCC effector cell function of peripheral blood mononuclear cells, measured by (51)Cr release assay, correlated inversely with viral load (R = -0.42, p = 0.007) and directly with CD4+ cell count (R = 0.52, p = 0.001). On the other hand, HIV-1-specific ADCC antibody level correlated directly with viral load, but only among patients with high CD4+ cell counts. Therapy-induced changes in ADCC effector cell function correlated strongly with changes in CD4+ cell count (R = 0.86, p = 0.014), whereas there was no consistent pattern of change in ADCC antibody with therapy. In a novel assay, ADCC reduced virus yield from CD4+ lymphocytes infected with a primary HIV isolate. ADCC may contribute to control of viremia, and CD4+ lymphocytes likely play a role in ADCC effector and antibody functions.

Comparison of Measles Virus-Specific Antibodies with Antibody-Dependent Cellular Cytotoxicity and Neutralizing Functions

Measles virus-specific antibodies with antibody-dependent cellular cytotoxicity (ADCC) and neutralizing functions were compared in 25 healthy women. ADCC antibody was measured in a 12-h 51Cr release assay using peripheral blood mononuclear cells as effectors and Raji cells persistently infected with measles as targets. Neutralizing antibody was determined by the plaque neutralization test (PNT). ADCC and PNT titers correlated well (r = .80, P < .001). ADCC titers ranged from < 10 to > 10(6) and averaged 1.3 logs higher than PNT titers. Three sera with PNT titers of < 120, including 1 with a titer of < 8 (< 10(0.9)), had ADCC titers of > or = 10(2.5). Among subjects born in the United States, ADCC titers, but not PNT titers, correlated inversely with the year of birth, suggesting that ADCC antibody may be more indicative than neutralizing antibody of differences between naturally and vaccine-acquired immunity. These data suggest a possible role for ADCC in protection or recovery from measles.

Role of Antibody-Dependent Cellular Cytotoxicity and Lymphokine-Activated Killer Cells in AIDS and Related Diseases

This overview summarizes current knowledge on the overall efficacy and potential contribution of antibody-dependent cellular cytotoxicity (ADCC) and lymphokine-activated killer cell (LAK) activities in evoking non-major histocompatibility complex (non-MHC) cytolytic responses to human immunodeficiency virus-1 (HIV-1)-infected targets. High titers of ADCC antibodies to the HIV-1 virion are present in HIV-1-seropositive populations at all stages of disease. These antibodies are broadly reactive with a large number of HIV-1 strains and are predominantly directed against envelope determinants spanning both gp120 and gp41. However, the relative ability of natural killer (NK) effectors, derived from HIV-seropositive individuals, to evoke ADCC responses becomes increasingly impaired with disease progression. HIV-1-seropositive individuals also show marked decreases in both production of and responsiveness to interleukin-2 (IL-2). HIV-1-seropositive individuals generally have the ability to generate ex vivo propagated LAK cells; however, these cytolytic effectors are less effective than their counterparts derived from healthy controls. Increased understanding and control of non-MHC-restricted cytotoxic-responses to HIV, and their induction by lymphokines, may lead to improved treatment strategies for the management of AIDS and related diseases.

Role of antibody-dependent cellular cytotoxicity in neonatal infection with herpes simplex virus.

The role of antibody and, in particular, antibody-dependent cellular cytotoxicity (ADCC) antibodies, in preventing or reducing the severity of infection with herpes simplex virus (HSV) in neonates is controversial. We have shown that human and murine neonates, in contrast with adults, are relatively deficient in ADCC leukocyte effector cell function. In an adoptive transfer model, the combination of ADCC-active human leukocytes and antibody to HSV could protect neonatal mice from lethal infection with HSV. Use of cells defective in ADCC function (e.g., from human neonates or patients with the CD11,18 deficiency in cell surface adhesive integrin) could not provide protection in this model. Finally, in human neonates the level of ADCC antibody at the time of infection with HSV was associated with severity of illness. Thus, ADCC is an important host defense against neonatal infection with HSV.

Primary Cytotoxicity against the Envelope Glycoprotein of Human Immunodeficiency Virus-It Evidence for Antibody-Dependent Cellular Cytotoxicity In Vivo

Fresh peripheral blood mononuclear cells from human immunodeficiency virus-1 (HIV-1) seropositive donors can lyse target cells expressing the envelope glycoprotein in vitro. In most cases, this antigen-specific lysis is not mediated by T lymphocytes. Lymphoblastoid cell lines infected with recombinant vaccinia viruses expressing different forms of the envelope protein of HIV-1 were used as target cells in chromium-release assays of primary cytotoxic effector cells and of antibody-dependent cellular cytotoxicity (ADCC). By depleting effector cells of CD16+ lymphocytes, or by blocking target cell lysis with an anti-human IgG serum, primary env-specific lysis was found to be due to ADCC, the effector cells being armed in vivo with specific, cytophilic antibodies. This phenomenon is dependent on cell surface expression of the envelope protein and is directed against both gp120 and gp41. Human HIV-1 antibody-positive sera are able to mediate ADCC against HIV-infected CD4+ T lymphocytes, suggesting a possible role of ADCC in the natural infection.

Different membrane anchors of Fc gamma RIII (CD16) on K/NK-lymphocytes and neutrophils. Protein- vs lipid-anchor.

Fc gamma RIII (CD16), the type three receptor for the Fc portion of IgG, is expressed on neutrophils, killer (K)/NK lymphocytes and macrophages. K/NK lymphocyte Fc gamma RIII, which plays a role in antibody-dependent cellular cytotoxicity, is an efficient signal transducing molecule, whereas neutrophil Fc gamma RIII, which plays a role in immune-complex clearance, seems less efficient in signal transduction. Neutrophil Fc gamma RIII has been reported to be a glycan-phosphatidylinositol-anchored membrane protein. Our studies suggest that K/NK lymphocyte Fc gamma RIII is protein-anchored rather than glycan-phosphatidylinositol-anchored. That is, K/NK lymphocyte Fc gamma RIII was resistant to phosphatidylinositol-specific phospholipase C and surface expression of Fc gamma RIII was not affected on K/NK lymphocytes from patients with paroxysmal nocturnal hemoglobinuria, a disorder of hemopoietic stem cells resulting in deficient expression of glycan-phosphatidylinositol-anchored proteins. Different membrane anchoring mechanisms of the Fc gamma RIII may account for different consequences of the ligand binding to two cell types.

Enhanced tumor growth and metastatic spread of an MH134 variant lacking a part of the MM antigen: a possible role of antibody-dependent cellular cytotoxicity in control of tumor growth and metastases.

The role of antibody-dependent cellular cytotoxicity (ADCC) in host defense against tumor growth and metastasis was investigated with MH134, an MM antigen-positive murine hepatoma, and MH134-M, a variant with enhanced tumorigenesis and metastasis, in syngeneic C3H/HeN mice. When inoculated subcutaneously into C3H/HeN mice, MH134-M tumors grew more rapidly than did MH134 tumors and consistently metastasized to the draining lymph nodes, whereas MH134 tumors did not. Also, MH134-M exhibited a significantly greater lung colonization potential than did MH134, when inoculated intravenously into C3H/HeN mice. In BALB/c nu/nu mice, however, solid MH134 tumors grew and metastasized to the same extent as MH134-M, indicating that there is no significant intrinsic difference between these two tumor lines in proliferative or metastatic capacity. Enzyme-linked immunosorbent assay (ELISA) and immunoblotting, performed after SDS-PAGE analysis of cellular extracts with a monoclonal antibody (MAb) that recognizes a part of the MM-antigen, revealed that the cells of MH134-M lack at least a part of the MM antigen. Sera of C3H/HeN mice bearing solid MH134 tumors were found to contain anti-MM-antigen antibodies, when tested by immunoblotting of SDS-PAGE-developed materials. Cytotoxicity testing in which thioglycollate-induced peritoneal macrophages were used as effector cells revealed that antibodies present in sera strongly induced ADCC to MH134 but not to MH134-M. On the other hand, sera of MH134-M tumor-bearing C3H/HeN mice neither contained anti-MM-antigen antibodies nor induced ADCC to MH134 or MH134-M tumor cells. Intravenous injection of carrageenan into C3H/HeN mice bearing solid MH134 tumors significantly enhanced tumor growth, whereas the growth of subcutaneously injected MH134-M tumors was not influenced by this treatment. These results suggest that the enhanced tumorigenesis and metastasis of the MH134-M line in C3H/HeN mice are based, at least in part, on significant loss of the MM antigen and the resultant inability to induce ADCC-triggering antibody production during tumor growth.

Human renal allograft rejection is predicted by serial determinations of antibody-dependent cellular cytotoxicity.

The early recognition and prompt treatment of rejection may minimise damage to renal allografts. Preliminary studies showed whole blood effector cell function in a constant antibody-dependent cellular cytotoxicity (ADCC) assay system to be suppressed in recipients with stable renal function. To investigate the possible role of ADCC in the rejection process, serial estimates were performed in 29 consecutive recipients of cadaveric kidneys. The generation of ADCC, as measured in vitro, preceded the biochemical confirmation of an impending rejection episode by 3 to 5 days for 31 of 33 episodes experienced by 23 recipients. Statistical analysis (X2) demonstrated a highly significant correlation between ADCC generation as measured in vitro and subsequent episodes of graft rejection. In contrast, six recipients who did not experience rejection episodes during the first 3 months postgrafting showed no evidence of ADCC activity. Thus, ADCC may be used to identify rejection earlier than has previously been possible.

Characteristics of isolated intestinal mucosal lymphoid cells in inflammatory bowel disease

Immunologic characteristics of intestinal mucosal lymphoid cells from patients with inflammatory bowel disease and controls have been compared. Mononuclear cells isolated by enzymatic means from intestinal tissues involved with inflammatory bowel disease were present in greater numbers, with increased proportions of macrophages and B-lymphocytes, particularly cells bearing intrinsic membrane immunoglobulin G. Synthesis of immunoglobulin G, measured by radioimmunoassay, was increased tenfold in inflammatory bowel disease, while immunoglobulin A synthesis per 106 cells was unchanged. “Null” or K-lymphocytes were absent from all populations, and antibody-dependent cellular cytotoxicity (a K-cell-mediated function) was not demonstrable. Taken together, the results fail to support a role for antibody-dependent cellular cytotoxicity or a defect in secretory immunoglobulin A, but rather focus attention upon possible forms of immunoglobulin G-mediated tissue damage in the pathogenesis or perpetuation of inflammatory bowel disease.