Role of polymorphic Fc receptor Fc gammaRIIa in cytokine release and adverse effects of murine IgG1 anti-CD3/T cell receptor antibody (WT31)

Rituximab-induced Cytokine Storm in the Absence of Overt Lymphoproliferative Disease

Rituximab is a monoclonal antibody that first demonstrated efficacy in the treatment of lymphoma but has since seen a dramatic growth in its use for other conditions. Cytokine release syndrome (CRS) is a rare but potentially fatal complication of rituximab infusion that has been described in patients with bulky lymphoproliferative disease. Here we report a convincing case of CRS occurring in a patient with no demonstrable lymphoproliferation. This case has implications for our understanding of the pathogenesis of CRS, our attempts to define an at-risk population and the design of future monoclonal antibodies.

Development of a cell-based assay measuring the activation of FcγRIIa for the characterization of therapeutic monoclonal antibodies

Antibody-dependent cellular cytotoxicity (ADCC) is one of the important mechanisms of action of the targeting of tumor cells by therapeutic monoclonal antibodies (mAbs). Among the human Fcγ receptors (FcγRs), FcγRIIIa is well known as the only receptor expressed in natural killer (NK) cells, and it plays a pivotal role in ADCC by IgG1-subclass mAbs. In addition, the contributions of FcγRIIa to mAb-mediated cytotoxicity have been reported. FcγRIIa is expressed in myeloid effector cells including neutrophils and macrophages, and it is involved in the activation of these effector cells. However, the measurement of the cytotoxicity via FcγRIIa-expressing effector cells is complicated and inconvenient for the characterization of therapeutic mAbs. Here we report the development of a cell-based assay using a human FcγRIIa-expressing reporter cell line. The FcγRIIa reporter cell assay was able to estimate the activation of FcγRIIa by antigen-bound mAbs by a very simple method in vitro. The usefulness of this assay for evaluating the activity of mAbs with different abilities to activate FcγRIIa was confirmed by the examples including the comparison of the activity of the anti-CD20 mAb rituximab and its Fc-engineered variants, and two anti-EGFR mAbs with different IgG subclasses, cetuximab (IgG1) and panitumumab (IgG2). We also applied this assay to the characterization of a force-oxidized mAb, and we observed that oxidation significantly decreased the FcγRIIa activation by EGFR-bound cetuximab. These results suggest that our FcγRIIa reporter assay is a promising tool for the characterization of therapeutic mAbs, including Fc-engineered mAbs, IgG2-subclass mAbs, and their product-related variants.

Correlation of ADCC activity with cytokine release induced by the stably expressed, glyco-engineered humanized Lewis Y-specific monoclonal antibody MB314

A major limitation to the application of therapeutic monoclonal antibodies (mAbs) is their reduced in vivo efficacy compared with the high efficacy measured in vitro. Effector functions such as antibody-dependent cell-mediated cytotoxicity (ADCC) are dramatically reduced in vivo by the presence of high amounts of endogenous IgG in the serum. Recent studies have shown that modification of the glycosylation moieties attached to the Fc part of the mAb can enhance binding affinity to FcγRIIIα receptors on natural killer cells and thus may counteract the reduced in vivo efficacy. In the present study, a humanized IgG1/κ monoclonal antibody recognizing the tumor-associated carbohydrate antigen Lewis Y was stably produced in a moss expression system that allows glyco-engineering. The glyco-modified mAb (designated MB314) showed a highly homogeneous N-glycosylation pattern lacking core-fucose. A side-by-side comparison to its parental counterpart produced in conventional mammalian cell-culture (MB311, formerly known as IGN311) by fluorescence-activated cell sorting analysis confirmed that the target specificity of MB314 is similar to that of MB311. In contrast, ADCC effector function of MB314 was increased up to 40-fold whereas complement dependent cytotoxicity activity was decreased 5-fold. Notably, a release of immunostimulatory cytokines, including interferon γ, monocyte chemotactic protein-1 (MCP-1), interleukin-6 and tumor necrosis factor (TNF) was particularly induced with the glyco-modified antibody. TNF release was associated with CD14 (+) cells, indicating activation of monocytes.

Natural killer (NK): dendritic cell (DC) cross talk induced by therapeutic monoclonal antibody triggers tumor antigen-specific T cell immunity

Tumor antigen (TA)-targeted monoclonal antibodies (mAb), trastuzumab, cetuximab, panitumumab, and rituximab, have been among the most successful new therapies in the present generation. Clinical activity is observed as a single agent, or in combination with radiotherapy or chemotherapy, against metastatic colorectal cancer, head and neck cancer, breast cancer, and follicular lymphoma. However, the activity is seen only in a minority of patients. Thus, an intense need exists to define the mechanism of action of these immunoactive mAb. Here, we discuss some of the likely immunological events that occur in treated patients: antibody-dependent cellular cytotoxicity (ADCC), cross talk among immune cells including NK cells and dendritic cells (DCs), and generation of TA-specific T lymphocyte responses. We present evidence supporting the induction of "NK:DC cross talk," leading to priming of TA-specific cellular immunity. These observations show that mAb-mediated NK cell activation can be greatly enhanced by the action of stimulatory cytokines and surface molecules on maturing DC and that NK:DC interaction facilitates the recruitment of both NK cells and DC to the tumor site(s). The cooperative, reciprocal stimulatory activity of both NK cells and DC can modulate both the innate immune response in the local tumor microenvironment and the adaptive immune response in secondary lymphoid organs. These events likely contribute to clinical activity, as well as provide a potential biomarker of response to mAb therapy.

Future therapeutics for the induction of peripheral immune tolerance in autoimmune disease and organ transplantation

Rodent models of transplantation and autoimmune disease have demonstrated that it is possible to induce lifelong and specific immunological tolerance to both self and graft antigens in the absence of any continued immunosuppression. If this situation could be achieved clinically, it would avoid many of the longer-term complications of immunosuppression, such as the increased risk of infection, cancer and other side effects, such as nephrotoxicity. In this review, we shall consider the interplay between regulatory T cells, dendritic cells and the tissue itself, and the resulting local protective mechanisms that are coordinated to maintain the tolerant state and an acquired local immune privilege. The current status of attempts to apply tolerogenic approaches to the clinical treatment of autoimmune diseases and to induce either tolerance to organ grafts or sufficient immune regulation so that conventional immunosuppression can be minimized will also be considered.

Tumor antigen-targeted, monoclonal antibody-based immunotherapy: clinical response, cellular immunity, and immunoescape

PURPOSE

Tumor antigen (TA) -targeted monoclonal antibodies (mAb), rituximab, trastuzumab, and cetuximab, are clinically effective for some advanced malignancies, especially in conjunction with chemotherapy and/or radiotherapy. However, these results are only seen in a subset (20% to 30%) of patients. We discuss the immunologic mechanism(s) underlying these clinical findings and their potential role in the variability in patients' clinical response.

METHODS

We reviewed the evidence indicating that the effects of TA-targeted mAb-based immunotherapy are mediated not only by inhibition of signaling pathways, but also by cell-mediated cytotoxicity triggered by the infused TA-targeted mAb. We analyzed the immunologic variables that can influence the outcome of antibody-dependent cell-mediated cytotoxicity (ADCC) in vitro and in animal model systems. We also analyzed the correlation reported between these variables and the clinical response to mAb-based immunotherapy.

RESULTS

Of the variables that influence ADCC mediated by TA-targeted mAb, only polymorphisms of Fcγ receptors (FcγR) expressed by patients' lymphocytes were correlated with clinical efficacy. However, this correlation is not absolute and is not observed in all malignancies. Thus other variables may be responsible for the antitumor effects seen in mAb-treated patients. We discuss the evidence that triggering of TA-specific cellular immunity by TA-targeted mAb, in conjunction with immune escape mechanisms used by tumor cells, may contribute to the differential clinical responses to mAb-based immunotherapy.

CONCLUSION

Identification of the mechanism(s) underlying the clinical response of patients with cancer treated with TA-targeted mAb is crucial to optimizing their application in the clinic and to selecting the patients most likely to benefit from their use.

Monoclonal antibody-induced cytokine-release syndrome

Monoclonal antibodies (mAbs) are widely used in anti-inflammatory and tumor therapy. Although effective, mAbs can cause a variety of adverse effects. An important toxicity seen with a few mAbs is cytokine-release syndrome (CRS). These mAbs include: alemtuzumab, muromonab-CD3, rituximab, tosituzumab, CP-870,893, LO-CD2a/BTI-322 and TGN1412. By contrast, over 30 mAbs used clinically are not associated with CRS. In this review, the clinical aspects of CRS, the mAbs associated with CRS, the cytokines involved and putative mechanisms mediating cytokine release will be discussed. This will be followed by a discussion of the poor predictive value of studies in animals and the prospects for creating in vitro screens. Finally, approaches to decreasing the probability of CRS, decreasing the severity or treating CRS, should it occur, will be described.

Immunotherapy of head and neck cancer using tumor antigen-specific monoclonal antibodies

Monoclonal antibodies (mAbs) are now commonly used therapeutic agents in cancer patients. Since US Food and Drug Administration approval of cetuximab for head and neck squamous cell carcinoma, it has been used increasingly in this disease. Several other mAbs also are in development or in clinical -trials. Recently, evidence has accumulated that mAbs induce activation of cellular immunity, including natural killer and T cells and that this may contribute to clinical response. mAbs have been shown to mediate antibody-dependent cellular cytotoxicity, complement-dependent lysis, and activation of tumor antigen-specific T cells. Various patient and tumor factors, such as polymorphisms in Fcgamma receptors expressed by immune cells, activity of T-regulatory cells, and tumor escape through downregulation of antigen-processing machinery in tumor cells, are likely to modulate the immune activation mediated by therapeutic mAbs. Understanding the interplay of these factors is likely to improve the selection of the most appropriate candidates for mAb-based immunotherapy, prediction of clinical response, and our understanding of mechanisms of tumor escape from therapeutic mAbs.

Biologic therapies in rheumatology: lessons learned, future directions

During the past decade biologic therapies such as monoclonal antibodies and fusion proteins have revolutionized the management of rheumatic disease. By targeting key cytokines and immune cells biologics have provided more specific therapeutic interventions with less immunosuppression. Clinical use, however, has revealed that their theoretical simplicity hides a more complex reality. Efficacy, toxicity and even pharmacodynamic effects can deviate from those predicted, as poignantly illustrated by the catastrophic effects witnessed during the first-into-human administration of TGN1412. This review summarizes lessons gleaned from practical experience and discusses how these can inform future discovery and development of new biologic therapies for rheumatology.

Mac-1 promotes FcgammaRIIA-dependent cell spreading and migration on immune complexes

The integrin Mac-1 plays a critical role in Fc receptor (FcR)-mediated antibody-dependent cellular cytotoxicity (ADCC). However, the mechanism by which Mac-1 facilitates the functions of FcgammaRIIA, a major FcR expressed on human leukocytes, is not fully understood. We report here that Mac-1 sustains cell adhesion, enhances cell spreading, and accelerates cell migration on preformed immune complexes (ICs) by directly interacting with FcgammaRIIA but not with the IC substrate. Coupling Mac-1 to FcgammaRIIA allows FcgammaRIIA to reside in the leading front of actin polymerization at the filopodial extension and thus could potentially enhance FcgammaRIIA-mediated cell spreading and migration. The direct interaction between Mac-1 and FcgammaRIIA is demonstrated by co-immunoprecipitation, by cell surface co-localization, and by solid-phase binding assays using recombinant alpha(M)I-domain and soluble FcgammaRIIA. Further mutational analysis identifies the E(253)-R(261) sequence within the alpha(M)I-domain as part of the FcgammaRIIA binding interface within Mac-1. Altogether, these results demonstrate that FcgammaRIIA recognizes Mac-1 via the alpha(M)I-domain but not the lectin domain, a distinct feature from other FcRs, and that Mac-1 binding confers FcgammaRIIA with the ability to prolong cell adhesion as well as to spread and migrate on the ICs, leading to effective cell killing by ADCC.

FCGR2A Polymorphism Is Correlated With Clinical Outcome After Immunotherapy of Neuroblastoma With Anti-GD2 Antibody and Granulocyte Macrophage Colony-Stimulating Factor

Purpose

Anti-GD2 murine IgG3 antibody 3F8 kills neuroblastoma cells by antibody-dependent cell-mediated cytotoxicity (ADCC). Granulocyte macrophage colony-stimulating factor (GM-CSF) enhances phagocyte-mediated ADCC. The differential affinity of the human FCGR polymorphic alleles for 3F8 may influence the effectiveness of antibody immunotherapy.

Patients and Methods

The entire cohort of high risk neuroblastoma patients (N = 136) treated on protocol using 3F8 and GM-CSF were the subjects of this analysis. Tumor response was measured by standard clinical tools plus sensitive molecular monitoring using quantitative reverse transcription-polymerase chain reaction (qRT-PCR). Polymorphic alleles of FCGR2A and FCGR3A were determined by PCR plus direct sequencing using genomic DNA samples obtained from marrow or blood of patients.

Results

FCGR2A (R/R) genotype correlated with progression-free survival for the entire cohort (P = .049) and for the subset of patients with no history of prior relapse (P = .023). FCGR2A (R/R) also correlated with marrow remission 2.5 months after treatment initiation: by histology (P = .021 and P = .036, for the entire cohort and the subset, respectively) and by qRT-PCR (P = .052 and P = .033, respectively).

Conclusion

The favorable outcome associated with FCGR2A (R/R) genotype is consistent with the proposed role of FCGR2A and phagocyte-mediated ADCC in 3F8 plus GM-CSF immunotherapy.

The FcgammaRIIa polymorphism in patients with chronic kidney graft rejection

The FcgammaRIIa receptors, which provide a crucial link between cellular and humoral components of the immune response, display allelic polymorphism. Individuals are homozygous for either arginine 131 (RR131) or histidine 131 (HH131) or are heterozygous for these two alleles (RH131). The HH131 genotype binds human IgG2 with high RR131 with low, and RH131 with intermediate affinity. The aim of the study was to evaluate the FcgammaRIIa polymorphism in patients with chronic kidney graft rejection. The study included 121 renal transplant recipients: 53 patients with long-term stable graft function and 68 with chronic allograft rejection. The distribution of FcgammaRIIa genotypes in patients with chronic kidney graft rejection did not differ significantly from that in patients with stable graft function. The results suggest that the FcgammaRIIa polymorphism is not an important genetic risk factor for chronic rejection of kidney allografts.

Flow cytometric determination of FcγRIIa (CD32) polymorphism

A guanine to adenine point mutation results in an arginine (R) to histidine (H) substitution in FcgammaRIIa at residue 131 that strongly impacts receptor function. This FcgammaRIIa polymorphism is mostly typed by allele-specific polymerase chain reactions (PCR) or in functional assays, dependent on ligand binding. Both types of methods are laborious, time consuming, and not readily available in routine laboratories. We generated a panel of human antibodies against FcgammaRII, and one of them, MDE-9, selectively recognized the FcgammaRIIa-H131 allotype. MDE-9 was applicable to detect FcgammaRIIa-H131 in both flow cytometry and immunohistochemistry. MDE-9 was used to develop an FcgammaRIIa allotyping method based on flow cytometry. In a "single-tube assay", FITC-labeled MDE-9 (specific for FcgammaRIIa-H131) and Cy3-labeled mAb 41H16 (specific for FcgammaRIIa-R131) were added to 50 mul samples of whole blood. The results of flow cytometric FcgammaRIIa allotyping correlated completely with PCR genotyping. This novel allotyping assay should facilitate the screening of patients in a routine diagnostic setting. In addition, a combination of MDE-9 and 41H16 can be used in FcgammaRIIa-H/H131 homozygous individuals to detect FcgammaRIIa and FcgammaRIIb surface expression on monocytes. This is an important application of these antibodies because, to this day, no antibodies were available to specifically study the surface expression of FcgammaRIIb.

Association of Fc Gamma Receptor IIA Polymorphisms with Acute Renal-Allograft Rejection

Acute rejection is a leading cause of early renal-allograft failure. The human Fc gamma receptor IIA (FcgammaRIIA) forms an essential link between the humoral branch and the effector cells of the immune system. In this study, we examined FcgammaRIIA genotypes in renal-allograft recipients (rejectors) with acute graft rejection and in a number of control groups to investigate a possible association between FcgammaRIIA polymorphism and acute renal-allograft rejection. The distribution of the genotypes in the study patient group differed from the control groups. The frequency of homozygosity for FcagammaRIIA-R/R131 in the rejectors was significantly higher than that in the recipients (nonrejectors) with well-functioning renal allografts and in blood donors (P< 0.05). In comparison with the control groups, the rejectors displayed a higher R131 allele frequency (P< 0.05) and a lower H131 allele frequency (P< 0.05). These results reveal a significant association between FcgammaRIIA-R/R131 and acute renal-graft rejection, and it is likely that FcgammaRIIA polymorphisms could be useful markers for potential risk of rejection.

FcγRIIIb Allele-Sensitive Release of α-Defensins: Anti-Neutrophil Cytoplasmic Antibody-Induced Release of Chemotaxins

Antineutrophil cytoplasmic Abs (ANCA) can activate neutrophils in an FcgammaR-dependent manner, but the link between this ANCA-induced effect and mononuclear cell activation with the characteristic granuloma formation of Wegener's granulomatosis is unclear. Human alpha-defensins, small cationic antimicrobial peptides, are found in neutrophils and have chemotactic activity for T cells, dendritic cells, and monocytes. In this study, we quantitated the release of alpha-defensins (human neutrophil peptides 1-3) from human neutrophils after targeted FcgammaR cross-linking (XL). Homotypic XL of FcgammaRIIa, FcgammaRIIIb, or heterotypic XL of both receptors resulted in significant release of alpha-defensins, an effect also induced by both human polyclonal and murine monoclonal cytoplasmic staining ANCA (anti-proteinase 3). This release of alpha-defensins, as well as of other granule constituents (ANCA targets anti-proteinase 3 and myeloperoxidase and elastase), was significantly greater in donors homozygous for the NA1 allele of FcgammaRIIIb than in donors homozygous for NA2. Interestingly, the ANCA-induced release was completely inhibited by the IgG Fc-binding peptide TG19320, which blocks the IgG-Fc region from binding to FcgammaR. Based on their chemotactic properties, alpha-defensins and their release by ANCA may contribute to modulation of the acquired immune response and to granuloma formation. The greater activity of the FcgammaRIIIB-NA1 genotype may also explain the greater severity of disease and its flare-ups in patients with this allele.

Variant genotypes of FcγRIIIA influence the development of Kaposi's sarcoma in HIV-infected men

Disturbances in inflammatory cytokine production and immune regulation coupled with human herpesvirus-8 (HHV-8) infection underlie the current understanding of the pathogenesis of Kaposi's sarcoma (KS), the most common HIV-associated malignancy. The low affinity Fc gamma receptors (FcγR) for IgG link humoral and cellular immunity by mediating interaction between antibodies and effector cells, such as phagocytes and natural killer cells. We examined the frequency of polymorphic forms of the low affinityFcγRs, FcγRIIA,FcγRIIIA, and FcγRIIIB in 2 cohorts of HIV-infected men with KS and found that theFcγRIIIA genotype exerts a significant influence on susceptibility to or protection from KS. The FF genotype was underrepresented in patients with KS, whereas the VF genotype was associated with development of KS. A similar association was observed between FcγRIIIA genotypes and HHV-8 seropositivity. These observations suggest a possible role forFcγRIIIA in the development of KS during HIV infection.

Variant genotypes of FcγRIIIA influence the development of Kaposi's sarcoma in HIV-infected men

Disturbances in inflammatory cytokine production and immune regulation coupled with human herpesvirus-8 (HHV-8) infection underlie the current understanding of the pathogenesis of Kaposi's sarcoma (KS), the most common HIV-associated malignancy. The low affinity Fc gamma receptors (FcγR) for IgG link humoral and cellular immunity by mediating interaction between antibodies and effector cells, such as phagocytes and natural killer cells. We examined the frequency of polymorphic forms of the low affinityFcγRs, FcγRIIA,FcγRIIIA, and FcγRIIIB in 2 cohorts of HIV-infected men with KS and found that theFcγRIIIA genotype exerts a significant influence on susceptibility to or protection from KS. The FF genotype was underrepresented in patients with KS, whereas the VF genotype was associated with development of KS. A similar association was observed between FcγRIIIA genotypes and HHV-8 seropositivity. These observations suggest a possible role forFcγRIIIA in the development of KS during HIV infection.

Different types of FCgamma-receptors are involved in anti-Lewis Y antibody induced effector functions in vitro

Stimulation of monocytes by interaction of monoclonal antibodies (mAbs) with Fc gamma receptors (FcgammaRs) results in the activation of various monocyte effector functions. In the present investigation we show that the anti-Lewis Y (LeY) anti-tumour mAb ABL 364 and its mouse/human IgG1 chimaera induce both antibody-dependent cellular cytotoxicity (ADCC) and the release of tumour necrosis factor alpha (TNF-alpha) during mixed culture of monocytes with LeY+ SKBR5 breast cancer cells in vitro. Although anti-LeY mAb-mediated TNF-alpha release paralleled ADCC activity, cytokine release required a higher concentration of sensitizing mAb than the induction of cytolysis. The determination of the FcgammaR classes involved in the induction of the distinct effector functions showed that anti-LeY mAb-induced cytolysis was triggered by interaction between anti-LeY mAbs and FcgammaRI. In contrast, mAb-induced TNF-alpha release mainly depended on the activation of monocyte FcgammaRII. Neutralization of TNF-alpha showed no influence on monocyte ADCC activity towards SKBR5 target cells. Our data indicate an independent regulation of anti-LeY mAb induced effector functions of ADCC and TNF-alpha release which seemed to be triggered by activation of different types of FcgammaR.

Induction of specific tolerance to allografts in rats by therapy with non-mitogenic, non-depleting anti-CD3 monoclonal antibody: association with TH2 cytokines not anergy

BACKGROUND

Anti-CD3 monoclonal antibodies (mAb) are potent immunosuppressives in transplantation but most do not induce tolerance. They induce anergy in Th1 cells but, if they bind to Fc receptors on antigen presenting cells, they activate T cells to release cytokines.

METHODS

This study examined the mechanisms of transplant tolerance induction to PVG fully allogeneic grafts in dark agouti rats by G4.18, a mouse immunoglobulinG3 anti-rat CD3 mAb that does not bind rat Fc receptors. Evidence of T cell activation was assayed by flow cytometry, reverse transcription (RT)-polymerase chain reaction (PCR) for cytokine mRNA, and responsiveness in mixed lymphocyte culture.

RESULTS

G4.18 treatment modulated T cell receptor/CD3 and CD2 and depleted T cells by <20% but did not induce activation surface markers. mRNA for interleukin (IL)-2, interferon (IFN)-gamma, tumor necrosis factor (TNF)-alpha, and IL-4 in the lymph node, spleen, and thymus was not increased, and IFN-gamma mRNA was reduced. G4.18-treated and naive rat cells had similar proliferation and expression of IL-2, IFN-gamma, and IL-4 in vitro. G4.18-treated allograft recipients had no induction of mRNA for IL-2, IFN-gamma, TNF-alpha, TNF-beta, IL-4, IL-5, IL-10, perforin, and granzyme A & B in the spleen or grafts, with levels similar to those in isografts. The IL-4 and IL-5 mRNA levels in the spleen but not the graft of G4.18-treated recipients were higher than in rejecting and naive animals. Cells from G4.18-treated graft recipients proliferated more rapidly to the donor than to the third party and had increased IL-4 expression.

CONCLUSIONS

G4.18 induced transplant tolerance by a combination of modulation and blocking of the TCR/CD3, associated with increased Th2 cytokines, without depletion, induction of anergy, or nonspecific activation of T cells.

Different CD3/T cell receptor monoclonal antibodies have distinct capacities to induce adhesion of T lymphocytes to endothelium

Murine CD3/T cell receptor (TCR) monoclonal antibodies (mAbs) induce immediate peripheral lymphocytopenias of different degree and duration. Lymphocytopenia is of short duration after the administration of immunoglobulin A CD3 mAb, but it persists much longer after the administration of immunoglobulin G2a CD3 mAb. Peripheral lymphocytopenia after the administration of WT31, a murine immunoglobulin G1 TCR mAb, appears to be dependent on the polymorphism of Fc(gamma)RIIa. In high responders, lymphocytopenia is comparable to that observed after immunoglobulin G2a CD3 mAb; in low responders, no lymphocytopenia occurs. In vitro, both immunoglobulin A and immunoglobulin G2a CD3 mAbs induce immediate activation of CD11a/CD18, with concomitant up-regulation of CD11b/CD18 on T cells, each of which is shown to be involved in the concurrent adhesion of T cells to endothelium. WT31 induces an immediate activation of CD11a/CD18 as well as T cell adhesion to endothelium in Fc(gamma)RIIa high responders only, interestingly without changes in the level of expression of CD11b/CD18. We conclude that the immediate occurrence of peripheral lymphocytopenia after the administration of CD3/TCR mAb is mediated by changes in the level of expression or avidity (or both) of adhesion molecules on T cells, whereas the persistence of this lymphocytopenia depends on the isotype of the CD3/TCR mAb and on the presence of suitable Fc receptors.