Fc Receptors For IgM Research Articles

Channel catfish cytotoxic cells: a mini-review

The use of allogeneic and autologous lymphoid cell lines has facilitated studies of cytotoxic T lymphocytes (CTL) and natural killer (NK)-like cells in channel catfish. Naı̈ve catfish leukocytes were shown to spontaneously kill allogeneic cells and virally-infected autologous cells without the need for prior sensitization, and allogeneic cytotoxic responses were greatly enhanced by in vitro alloantigen stimulation. Both catfish CTL and NK-like cells have been successfully cloned from these alloantigen-stimulated cultures, and represent the first cytotoxic cell lines derived from any ectothermic vertebrate. These cloned cytotoxic cells contain granules and likely induce apoptosis in sensititive targets via a putative perforin/granzyme mechanism. In addition, some catfish CTL clones may also kill targets by an additional mechanism, possibly by Fas/FasL-like interactions. Importantly, these cytotoxic cells do not express the marker for catfish nonspecific cytotoxic cells (NCCs), and thus represent cell types distinct from NCCs. The use of monoclonal antibodies against the catfish F and G immunoglobulin light chain isotypes revealed the presence of a putative Fc receptor for IgM (FcμR) on some catfish NK-like cells that appears to ‘arm’ these cells with surface IgM. In addition, a potentially important monoclonal antibody (CC41) developed against catfish NK-like cells was found to recognize an ~150 kDa molecule on the surface of catfish cytotoxic cells. These studies clearly demonstrate that catfish possess an array of different cytotoxic cells. The availability of various cloned cytotoxic cell lines should enable unambiguous functional studies to be performed in ways not currently possible with any other fish species.

A novel Fc receptor for IgA and IgM is expressed on both hematopoietic and non-hematopoietic tissues.

By contrast to well-defined Fc gamma and Fc epsilon receptors, the structural and functional characteristics of Fc mu receptor are unclear. We have recently described a novel mouse Fc receptor, designated Fc alpha/mu receptor, and its human homologue, which bind both IgM and IgA. Here we show that the Fc alpha/mu receptor is expressed on mature, but not immature, B lymphocytes and acquires the ability to bind IgM and IgA antibodies after stimulation of B lymphocytes. Moreover, stimulation with phorbol 12-myristate 13-acetate increased endocytosis of IgM-coated microparticles mediated by the Fc alpha/mu receptor expressed on pro-B cell line Ba/F3 cells. We also show that the Fc alpha/mu receptor is expressed in secondary lymphoid organs, such as lymph node and appendix, kidney and intestine, suggesting an important role of the receptor for immunity in these organs.

Effect of in vitro and in vivo migration of bovine neutrophils on binding and expression of Fc receptors for IgG2 and IgM

Summary Binding of endogenous and exogenous homologous IgG2 and IgM to bovine neutrophils before and after in vitro migration through micropore filters, and in vivo migration through mammary tissues after intramammary injection of endotoxin was evaluated by use of flow cytometry. Immunoglobulin binding to neutrophils at 4 and 37 C was also evaluated. Before and after in vitro migration, neutrophils with endogenously bound IgG2 and IgM averaged 1 and 2% and 23 and 7%, respectively. Before and after in vivo migration, IgG2 and IgM binding averaged 1 and 7% and 26 and 15%, respectively. Before and after in vitro migration, binding of purified IgG2 and IgM averaged 75 and 67% and 8 and 24%, respectively. Before and after in vivo migration, percentage of neutrophils binding purified IgG2 and IgM averaged 92 and 98% and 54 and 70%, respectively. When serum was used as a source of exogenous immunoglobulins, binding of total IgG after in vitro migration increased from 5% to 28% and of IgM from 4% to 20%. After in vivo migration, binding increased from 21% to 47% and from 24% to 56%, respectively. Exogenous binding of IgG2 at 4 and 37 C averaged 75 and 84%, and binding of IgM averaged 8% at either temperature. Endogenous IgG2 was unaffected by temperature; however, binding of IgM decreased from 23% at 4 C to 2% at 37 C. These data indicate that endogenous binding was higher for IgM before migration than after migration, in vitro and in vivo. Furthermore, migration in vivo through cellular matrices induced receptor upregulation for IgG and IgM. Source and concentration of ligand and serum components, other than immunoglobulins, appeared to contribute to receptor expression and availability. Neutrophils that were exposed to endotoxin and migrated into milk expressed more receptors than did unstimulated and nonmigrating neutrophils. The association of IgM with its receptor was temperature-dependent.

Expression of IgA and IgM Fc receptors on murine T lymphocytes.

Fc receptors are induced on T cells following activation via the TCR. T cells that express Fc receptors transiently have the ability to use two different cognate systems: the TCR and immunoglobulins bound to the Fc receptors. The studies discussed in this article are focused on the Fc alpha and Fc mu receptors that can be induced on certain subsets of murine T lymphocytes. The article emphasizes the role of the T cell receptor for antigen in the expression of Fc alpha and Fc mu receptors on murine T cells and reviews experimental observations that suggest significant molecular heterogeneity of these Fc receptors. The finding that regulation of expression of Fc alpha receptors and Fc mu receptors on T lymphocytes is linked to cellular activation via the CD3/TCR complex implies that these Fc receptors might mediate important functions in the biology and pathology of T cells.

The inhibition of T cell proliferative responses by crosslinking CD7 and IgM-Fc receptors

Previous work from our laboratory described a human T cell soluble ligand that inhibited T cell proliferative responses to mitogen and alloantigen by interacting with CD7 and/or the receptor for the IgM-Fc portion (FcRμ) on T cells. In this report, we used mouse anti-human CD7 monoclonal antibodies (mAb) and purified human IgM (HIgM) to substitute for the human ligand and examined the possible involvement of these receptors in the inhibition of T cell proliferation. Preincubation of human T cells with mouse anti-CD7 mAb, HIgM, mouse anti-human IgM (MAH IgM) alone, or any of these combinations as a primary antibody did not inhibit mitogenor alloantigen-induced T cell replication. Similar effects were seen with the pretreatment of T cells with an irrelevant negative control primary mAb or a secondary-step goat anti-mouse immunoglobulin (GAM Ig), goat anti-human IgM-Fc (GAH Fcμ), or both. In contrast, the pretreatment of T cells with anti-CD7 and/or HIgM followed by the appropriate secondary-step crosslinking antibody significantly reduced their proliferative responses to mitogen and alloantigen. Similarly, crosslinking of CD7 and FcRμ on human transformed T cell lines inhibited their spontaneous proliferation. The inhibitory effect of crosslinking CD7 and FcRμ was not due to cytotoxic effects of these antibodies and appears to be temperature sensitive. These findings suggest that crosslinking CD7 and/or FcRμ appears to have a novel role in down-regulating T cell proliferation.

Signal transduction through crosslinking CD7 and IgM-Fc receptors that inhibits T-cell proliferation

We have previously suggested a possible involvement of CD7 and/or the putative receptor for IgM-Fc (FcRμ) in the downregulation of T-cell responses to mitogen and alloantigen. In this report, we examined the effect of ligand-receptor interaction upon T-cell proliferation by using anti-CD7 monoclonal antibodies (mAbs) and purified human IgM (HIgM) and showed that crosslinking CD7 and/or FcRμ on T cells resulted in significant inhibition of mitogen- and alloantigen-induced proliferative responses. In most cases, crosslinking of FcRμ alone or together with CD7 receptors on peripheral blood lymphocytes (PBLs) resulted in more potent suppression of T-cell proliferation than that caused by crosslinking CD7 alone. Examination of potential inhibitory mechanisms revealed that crosslinking CD7 and FcRμ does not reduce cell viability or the expression of T-cell markers that are important for T-cell activation or proliferation. However, crosslinking CD7 and/or FcRμ of PBLs significantly reduced phytohemaglutinin (PHA)-induced IL-2 production and rendered PBLs unable to utilize exogenously added human recombinant IL-2 (rIL-2). Further examination of possible signal transduction that might be associated with crosslinking CD7 and/or FcRμ receptors indicated a marked reduction in the magnitude and duration of intracellular calcium (Ca ++)i released in response to PHA. These findings suggest that crosslinking CD7 and FcRμ inhibits T-cell activation and proliferation by a calcium-dependent mechanism that inhibits IL-2 production and/or utilization.

Molecular cloning of a novel member of the immunoglobulin gene superfamily homologous to the polymeric immunoglobulin receptor.

The CMRF35 monoclonal antibody recognizes a cell membrane antigen present on the surface of monocytes, neutrophils, a proportion of peripheral blood T and B lymphocytes and lymphocytic cell lines. Initial studies with CMRF35 suggested an unusual pattern of serological reactivity which did not correspond to any of the known leukocyte differentiation antigen clusters. We describe here the cloning and sequencing of a cDNA encoding the CMRF35 antigen by means of expression in COS cells and immunoselection with the CMRF35 monoclonal antibody. The cDNA encodes a novel integral membrane glycoprotein of 224 amino acids that represents a new member of the immunoglobulin (Ig) gene superfamily. The molecule comprises (a) a single extracellular Ig variable domain remarkably similar to the Fc receptor for polymeric IgA and IgM, (b) a membrane-proximal domain containing a high proportion of proline, serine and threonine residues that was predicted to be heavily O-glycosylated, (c) an unusual transmembrane anchor that contained a glutamic acid and a proline residue and (d) a short cytoplasmic tail. Transcripts encoding the CMRF35 protein were detected in early monocytic cell lines, in peripheral blood T cells and in some B lymphoblastoid cell lines, confirming the results of immunocytological staining. However, the level of CMRF35 expression on peripheral blood T cells was shown to decrease in response to mitogenic stimulation. The likelihood that the CMRF35 antigen shares a common evolutionary ancestor with the poly Ig Fc receptor is discussed.

In vitro and in vivo activation of murine gamma/delta T cells induces the expression of IgA, IgM, and IgG Fc receptors.

The present studies examined resting and activated murine gamma/delta T lymphocytes, in vitro and in vivo, for surface expression of FcR. Polyclonal gamma/delta TCR+ lymphocytes selectively grown from the spleen and intestine of normal mice did not express FcR when the cells were in a resting state, but when cells were activated with anti-CD3 antibody virtually all of the splenic gamma/delta lymphocytes and a large subpopulation of the intestinal gamma/delta lymphocytes expressed IgA and IgM FcR. This was confirmed by using transgenic mice. Resting gamma/delta TCR+ lymphocytes from the spleen, thymus, lymph node, and blood of gamma/delta TCR transgenic mice did not express FcR for any of the five major classes of Ig H chains. Activation of the gamma/delta TCR+ cells via the CD3/TCR complex induced high levels of IgM and IgA FcR and low levels of IgG FcR. Finally, in hepatic granulomas of schistosome-infected mice, activated gamma/delta TCR+ cells are present and express high levels of IgA and IgM FcR and low levels of IgG FcR. These investigations establish that transition of gamma/delta TCR+ lymphocytes from a resting to an activated state (triggered via the T3Ti TCR complex) is accompanied by the induction of surface membrane receptors specific for Ig H chain isotypes. The activation-linked expression of FcR on gamma/delta TCR+ lymphocytes provides potential mechanisms for coupling the functional activities of gamma/delta T lymphocytes with immune mechanisms that involve Ig molecules, such as antibody-dependent cellular cytotoxicity.

Expression of Fcμ receptors on human natural killer cells

Fc receptors for IgG (CD16) have been described as the only 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 (FcμR), 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 FcμR-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 FcμR 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.

若年性関節リウマチ患児における末梢血リンパ球subsetsに関する臨床的研究

The purpose of this investigation was to determine the proportion of IgM Fc receptor bearing T lymphocytes (Tμ) and IgG Fc receptor bearing T lymphocytes (Tγ) in the peripheral blood of patients with juvenile rheumatoid arthritis (JRA). Peripheral lymophocytes from 20 patients with JRA and 20 normal controls were studied. Patients with JRA were classified into active and inactive groups, with 8 patients in the former and 12 patients in the latter.In 4 patients, methylprednisolone was injected intravenously in doses of 30mg/kg over 1 hr for 3 days. Peripheral blood lymphocyte subsets were evaluated before receiving the pulse, as well as 24 hours after the start of the 3rd infusion.The proportion of peripheral blood Tγ cells in the inactive group was significantly higher than in the active group (p<0.01). The Tμ/Tγ ratio of peripheral blood lymphocyte in the active group was significantly higher than that in the inactive group (p<0.01).Moreover, the proportion of Tγ cells increased significantly after injection of methylprednisolone for 3 days (p<0.01).These findings suggest that the proportion of Tμ and Tγ in the peripheral blood lymphocytes is of benefit in monitoring disease activity and prognosis of disease in JRA.

A human suppressor T-cell factor that inhibits T-cell replication by interaction with the IgM-Fc receptor (CD7)

We have previously described the induction of human suppessor T cells from fresh peripheral blood lymphocytes of a kidney transplant recipient by in vitro stimulation with an autologous irradiated antidonor CTL line (EE-1) grown from a biopsy of the patient's own renal allograft. The induced T cells (designated TsEE) were shown to inhibit the in vitro generation of proliferative and cytotoxic responses of autologous T cells and nonautologous T cells that shared HLA-B7 with TsEE cells. Stimulation of TsEE cells by the autologous irradiated inducer line (EE-1) produced soluble factors (designated TsEEF) that similarly inhibited autologous and nonautologous T-cell responses to alloantigens and mitogens, but in a non-HLA-restricted manner. In this study, we examined the functional interaction of TsEEF with various cells surface receptors. TsEEF specifically inhibited the proliferation of stimulated and transformed T cells expressing CD7, a putative receptor for IgM-Fc (FcRμ). Blocking or capping of CD7-FcRμ determinants on responder T cells by pretreatment with IgM or anti-CD7 monoclonal antibodies (3A1, HuLy-m2) abrogated TsEEF activity. Conversely, pretreatment of T cells with TsEEF significantly reduced their binding of IgM and HuLy-m2. TsEEF was demonstrated not to be IgM or IgG, and its activity was not removed by preabsorption with IgM or IgG; however, its activity could be competitively inhibited by coculture with IgM. By cocapping experiments and studies utilizing CD7 − (Hut-78) and CD7 + (HSB, Molt-4) T-cell lines, TsEEF activity did not appear to involve interactions with other T-cell or non-T-cell surface receptors. These findings suggest a novel role for FcRμ-CD7 T-cell surface receptors in binding certain soluble T-cell factors that result in the inhibition of T-cell replication.

Fc receptors in corneal epithelium

The corneal epithelia of the mouse, rabbit and human were shown to contain Fc receptors by an indirect method with immunoglobulins, labelled second antibody and the avidin-biotin peroxidase complex (ABC); and a direct method with the peroxidase conjugated Fc fragment. The cornea epithelia of all three species exhibited a high concentration of the Fc gamma R with either homologous or heterologous immunoglobulins. The binding specificity of Fc receptors was further supported by competitive inhibition of binding of labeled antibody by homologous, unlabeled antibody. Although Fc alpha R was also present it was at a much lower concentration. The Fc receptor for IgM (Fc microR) was difficult to demonstrate. However, it appeared to be present in very low concentration on only the mouse and rabbit corneal epithelium.

Expression, distribution and specificity of Fc receptors for IgM on murine B cells.

Subpopulations of normal adult murine splenic B cells and a panel of murine B cell tumors were examined for their ability to bind murine IgM specifically. By using two-color flow cytometric analyses, we have demonstrated that 90 to 95% of surface (s)IgD+ B cells express surface membrane receptors for IgM (Fc mu R). The binding of pentameric murine IgM to splenocyte Fc mu R was IgM-specific since it was totally inhibited by other polymeric IgM proteins, but not by Ig of other H chain classes or by mAb specific for the murine IgG or IgE FcR. Binding of IgM to splenic cells was saturable. Fc mu R were co-expressed with the Fc gamma R as well as the Fc epsilon R on the majority of splenic B cells. Minor populations of splenic mononuclear cells expressed only an Fc mu R, Fc gamma R or Fc epsilon R. In a survey of B tumor cell lines representing different stages of B cell development, we observed that the Fc mu R was expressed on pre-B cell lines and that Fc mu R detection was maximal on immature B cell lines that expressed sIgM and low amounts of sIgD and Ia. Fc mu R were not detected on cell lines that had switched from sIgM to the expression of another sIg, or on plasmacytomas and hybridomas. The studies with normal splenocytes establish that the majority of sIgD+ B lymphocytes in adult BALB/c mice express surface membrane receptors that specifically bind IgM. The studies with B lineage tumor cells suggest that the expression of Fc mu R on B cells is developmentally regulated and that the pattern of expression exhibited by Fc mu R during B cell ontogeny differs from the patterns that have been previously found for IgG and IgE FcR. These observations raise the possibility that Fc mu R might have a functional significance in some aspect of B cell maturation and activation. By using a family of IgM H chain constant region domain deletional mutants, we have further demonstrated that, like the T cell Fc mu R, the B cell Fc mu R also requires a C mu 3 domain for binding to occur, raising the possibility that the T and B cell Fc mu R in mice may be structurally related molecules.

Fc receptor for shark IgM

Fc receptors for shark IgM have been demonstrated on shark leukocytes. Measurement of receptor binding required treatment of leukocytes with Cytochalasin D to inhibit phagocytosis. EA rosetting assays were carried out using human erythrocytes coated with shark anti-human antibody. Binding to shark leukocytes was demonstrated to be specific to shark IgM in that affinity purified shark IgM and purified Fc5μ fragments could block rosette formation, but not shark transferrin, bovine serum albumin or fetal bovine serum. The binding was shown to be saturable and reversible, characteristic of receptor-ligand interaction. Further, it was shown that affinity purified, radioiodinated IgM could also bind Cytochalasin D-treated shark leukocytes in a manner analogous to rosetting. We conclude that Fc receptors appeared early in evolution, and that previous difficulties in demonstrating the Fcμ receptor resulted from non-specific binding associated with phagocytosis.

Simultaneous expression of multiple immune complex receptors on murine thymocytes and spleen cells

Mixed rosette studies were performed to evaluate the coexpression of IgG Fe, IgM Fc, and complement receptors (C3R) by thymocytes obtained from mice 7 days after cortisone injection and by spleen cells. Indicator cells coated with IgM, IgG, or C3 independently were mixed and could be distinguished by morphology or by a fluorescein label. In double-marker studies, 36% of spleen cells formed rosettes with IgG-and/or IgM-sensitized red blood cells. Among this population there was a 24% overlap of cells binding IgM and IgG complexes simultaneously. Of the spleen cells, 78% bound IgM- and/or C3-sensitized cells. Of the spleen cells forming rosettes with IgM and C3 indicator cells, 15% coexpressed these receptors. With IgG and C3 indicator cells, 58% of spleen cells bound to one or both kinds of complexes with an 18% overlap. Of cortisone-resistant thymocytes, 14% formed rosettes with IgM- and/or IgG-sensitized red blood cells; within this population there was an overlap of 21%. With IgM- or C3-sensitized cells, 19% of cortisone-resistant thymocytes bound to one or both, among which there was a coexpression of 21%. With IgG- or C3-sensitized cells, there was a 14% overlap of rosette-forming cells binding both. In triple-marker studies 79% of spleen cells formed rosettes wit C3-, IgG-, and/or IgM-sensitized indicator cells, out of which 11% coexpressed IgM and IgG FcR, 20% coexpressed IgG and C3R, and 10% coexpressed IgM FcR and C3R. Of rosette-forming cells, 13% coexpressed all three receptors. With cortisone-resistant thymocytes, 19% bound one or more kinds of immune complexes. Among these, 9% coexpressed IgG FcR and C3R, 14% coexpressed IgM FcR and C3R, and 14% bound IgG and IgM complexes. We could not detect the simultaneous expression of all three receptors on cortisone-resistant thymocytes. Using Isopaque-Ficoll fractionation of cells binding C3-sensitized cells, cortisone-resistant thymocytes were enriched and depleted of C3-receptor-bearing cells and their Lyt phenotypes were determined by immunofluorescence microscopy. The C3-receptor-enriched population contained 56% C3R + cells which were 79% Lyt-1 positive and 100% Lyt-2 positive. The C3R-depleted population contained 1.3% C3R − cells with 10% Lyt-1 positive and 22% Lyt-2 positive among the total. Surface phenotypic expression of normal and cortisone-resistant thymocytes was also evaluated by direct and indirect fluorescence by fluorescence-activated cell sorter (FACS). Greater than 90% of the normal thymocyte population expressed Lyt-1, Lyt-2, L3T4, and thy-1 antigens whereas 80% of the cortisone-resistant population expressed Lyt-1 and thy-1 antigens and around 50% expressed Lyt-2 and L3T4 antigens.

Canine cyclic hematopoiesis: Alterations in T lymphocyte subpopulations in peripheral blood, lymph nodes, and thymus of gray collie dogs

Cyclic hematopoiesis (CH), also called cyclic neutropenia, is an inherited disorder known to occur in both humans and gray collie dogs. Previous reports have provided ample evidence to suggest that lymphocyte activity and regulatory mechanisms may be abnormal in CH. The present study examined the lymphocyte populations of several lymphoid compartments of gray collie dogs. The percentage of B lymphocytes in the lymph nodes of CH dogs was significantly increased whereas that of null lymphocytes was decreased. The percentage of T lymphocytes did not differ between CH and normal dogs, however, the proportions of T lymphocyte subpopulations were significantly different. The levels of T lymphocytes expressing IgGFc receptors (Tγ) in the thymus, lymph nodes, and peripheral blood were significantly increased; whereas the levels of T lymphocytes expressing IgMFc receptors (Tμ) were significantly decreased. The percentage and absolute numbers of Tγ and Tμ lymphocytes cycled in CH dogs. The percentage and absolute numbers of neutrophils were greatest when that of Tγ lymphocytes was reduced. The cycles of monocytes and Tγ lymphocytes occurred in close association and a linear relationship between the levels of these cells was observed both in terms of percentage ( r = 0.62; P < 0.01) and absolute number ( r = 0.67; P < 0.05). The percentage of Tγ and Tμ lymphocytes were inversely correlated ( r = − 0.68; P < 0.01).

The effect of bilirubin on the Fc receptor expression and phagocytic activity of mouse peritoneal macrophages.

The effect of bilirubin on the phagocytic activity of mouse peritoneal macrophages and on the expression of Fc receptors and receptors for SRBC was studied. Intraperitoneally administered bilirubin influenced the expression of Fc receptors for IgM, IgG2B, IgA and IgE, whereas the expression of other receptors as well as the phagocytic activity of peritoneal macrophages remained unchanged. The possible mechanism of the effect of bilirubin on Fc receptors is discussed.

Antibody-dependent cellular cytotoxicity of Trypanosoma theileri mediated by purified bovine isotypes and subisotypes.

Bovine neutrophils, eosinophils and macrophages mediated in vitro cytotoxicity against Trypanosoma theileri in the presence of purified IgM, IgG1 and IgG2 from immune bovine serum. When the immunoglobulin fractions were assayed at similar concentrations, IgM was the most effective isotype mediating killing with all three effector cell types. Using the ELISA with monospecific antisera against the different bovine isotypes and subisotypes, IgM was shown to be contaminated by less than 1%. The addition of 0.08 M 2-mercaptoethanol inhibited IgM-mediated ADCC but not that of IgG1 or IgG2, and the cytotoxicity occurred in the absence of complement. The presence of isotype and subisotype specific Fc receptors on the bovine effector cells was investigated using a totally homologous erythrocyte-antibody (EA) rosetting technique. FC receptors for bovine IgM, IgG1 and IgG2 were detected on bovine neutrophils. Very few FcM receptors were detected on either eosinophils or macrophages, but FcG2 receptors were detected on both cell types, and FcG1 receptors on macrophages. However, eosinophils showed very few FcG1 receptors. The failure to detect all types of Fc receptor on the three different effector cells is discussed.

Antigen-specific major histocompatibility complex-restricted helper T cell clones are sufficient to induce unprimed B cells to switch and to secrete IgG and IgA in a primary in vitro polyclonal response.

To investigate the role of helper T (Th) cells in the regulation of the production of the various immunoglobulin (Ig) classes and subclasses, we have used poly (Glu60 Ala30Tyr10) (GAT)-specific, major histocompatibility-complex-restricted Th cell clones to stimulate unprimed B cells. The T cells used in these studies were Thy-1+, Lyt-1+, Lyt-2- and lacked Fc receptor for IgM, IgG and IgA, and the unprimed splenic B cells were selected by the fluorescence-activated cell sorter for their lack of expression of surface (s)IgG and by panning for their lack of expression of sIgA. We have taken advantage of the ability of some antigen-specific major histocompatibility complex (MHC)-restricted Th cell clones to polyclonally activate unprimed B cells in vitro in the presence of high doses of antigen. We have shown that under these conditions, an antigen-specific MHC-restricted Th cell clone is sufficient to induce the switch of sIgG- sIgA- unprimed B cells to IgG and IgA, as well as the expansion of these cells and their differentiation into IgG and IgA-secreting cells. Isotype-specific Th cells thus do not seem to be an absolute requirement for the production of the various IgG subclasses and of IgA.

Dipeptidyl peptidase IV as a new surface marker for a subpopulation of human T-lymphocytes

Plasma membranes were isolated from normal human lymphocytes as well as from cells of chronic lymphocytic leukemia of the T type. In both cases the bulk of the dipeptidyl peptidase IV activity paralleled the distribution of 5′-nucleosidase and, therefore, was localized in the plasmalemma. Immunofluorescence experiments with normal human lymphocytes and with antibodies against dipeptidyl peptidase IV revealed that this peptidase was accessible on the surface of viable cells. Further, it was demonstrated that the relative number of dipeptidyl peptidase IV-positive cells is much higher in lymphocytes reacting with the OKT4 antibody than in OKT8-positive cells. On the other hand, it has been reported that this peptidase is absent in B lymphocytes and is predominantly found in T cells bearing the Fc receptor for IgM (Tμ lymphocytes). Thus, it is concluded that dipeptidyl peptidase IV represents an easily demonstrable surface marker of this lymphocyte subset.