Antigen presentation by B cells and its significance in T-B interactions

Requirement for noncognate interaction with T cells for the activation of B cell immunoglobulin secretion by IL-2

The mechanism whereby noncognate contact with activated IL-2-producing Type 1 helper T cells (TH1) induces B cell activation was examined. Small resting B cells from C57B1/6 mice were cultured, in the absence of any ligand for surface Ig, with irradiated cells of the hapten-specific, CBA-derived, F23.1+ TH1 clone E9.D4 in F23.1 (anti-T cell receptor V-beta 8)-coated microwells. This induced polyclonal B cell activation to enter cell cycle (thymidine incorporation) at 2 days and to secrete immunoglobulin at 5 days. An anti-IL-2 mAb (S4B6) inhibited antibody production completely. Anti-IL-2 did not inhibit either LPS-induced B cell responses, or T cell activation (measured as IL-3 secretion). Anti-IL-2 receptor (anti-Tac) mAbs also inhibited T-dependent B cell responses, without affecting LPS responses. An anti-IFN-gamma mAb partially inhibited Ig secretion, without affecting entry into cycle. LPS responses or T cell activation. Other antibodies (anti-IL-3, IL-4, IL-5, Thy-1.2, CD5) were not inhibitory. After 2 days of culture with F23.1-activated T cells, B cells appeared to have become responsive to IL-2, in that they could be driven to immunoglobulin production by the addition of IL-2. Flow cytometry showed no expression by these B cells of 55-kDa (Tac) IL-2 receptors. Also, rigorous removal of T cells from 2-day cocultures prevented the response to IL-2, and readdition of T cells restored it. Because the reconstituted responses were inhibited both by anti-IL-2 and by anti-Tac, IL-2 must have acted indirectly, via the T cells that were present in these cultures. Continued contact with T cells was therefore necessary for the progression of B cells to antibody secretion.

A role for adhesion molecules in contact-dependent T help for B cells

The role of cell contact in T-dependent B cell activation was examined. Small resting B cells from C57BL/6 mice were cultured with CBA-derived, non-alloreactive cloned T helper cells in anti-T cell receptor V beta 8-coated microwells. This induced polyclonal B cell activation to enter cell cycle (as measured by thymidine incorporation at 2 days) and to secrete immunoglobulin (as measured by an enzyme-linked immunoassay detecting high-rate Ig secretion at 5 days). The inclusion of monoclonal antibodies against LFA-1. ICAM-1 and CD4 in these cultures strongly inhibited antibody responses, although proliferative responses were only inhibited to about 50%. Inhibitory monoclonal antibodies did not significantly affect lipopolysaccharide-induced responses. T cell activation to interleukin (IL) 3 secretion, nor did they inhibit the formation of multicellular clusters containing T and B cells. There was no correlation between the level of expression of adhesion molecules by T cells and their ability to induce B cell responses. Anti-LFA-1 abrogated T-dependent responses to IL2 which were inducible after 2 days in culture, but did not inhibit the induction of this IL2 responsiveness. These results suggest that continued cell contact involving adhesion/accessory molecules induces B cells to proliferate and to respond to T cell lymphokines. A signaling role for cell interaction molecules on B cells is proposed, similar to the role of these and analogous molecules on T cells.

Mutations of immunoglobulin transmembrane and cytoplasmic domains: effects on intracellular signaling and antigen presentation

The membrane-bound form of immunoglobulin serves as an antigen-specific receptor for B cells mediating signal transduction and antigen presentation. We have developed an assay that reconstitutes both these physiologic responses with respect to the antigen phosphorylcholine. By introducing specific mutations in the human Ig mu chain gene, we have shown that certain transmembrane residues and the short cytoplasmic domain are crucial for these two activities. Moreover, elimination of a single transmembrane hydroxyl group severely inhibits antigen presentation without affecting signal transduction, suggesting that these two functions are mediated by different protein interactions.

Class II MHC molecules and antigen enter the same vesicles during internalization by resting B lymphocytes

Efficient presentation of Ag by a B cell to a T cell requires that Ag bind to the Ag receptor (Ig) on the B cell, after which it is internalized into an acid compartment where it is modified and returned to the cell surface in the context of class II MHC molecules. It remains uncertain whether processed Ag binds to class II which has been internalized and recycled with Ag, or to nascent class II inside the cell. To determine if cell surface class II enters the same vesicles as Ag, or is excluded during internalization of Ag which is bound to the B cell receptor, 5- and 16-nm gold particles were labeled with anti-class II and anti-Ig, respectively. Cells were incubated at 37 degrees C and internalization of these particles was observed using electron microscopy. By 10 min, 60-75% of the B cell sections contained vesicles with gold particles inside them. Between 40 and 64% of these vesicles had both 5- and 16-nm particles. Maximum internalization occurred by 30-60 min, and by 2 hr the number of small and large particles on the B cell surface became constant or increased, respectively. Both kinds of particles moved from electron-lucent to electron-dense vesicles as the incubation time increased, although a portion of the anti-class II particles remained in electron-lucent vesicles. These data clearly show that labeled, cell surface class II is not selectively excluded from Ag-containing vesicles during Ag internalization. Thus, cointernalization of Ag and class II may represent a mechanism by which processed Ag meets class II.

The role of B cells for in vivo T cell responses to a Friend virus-induced leukemia

B cells can function as antigen-presenting cells and accessory cells for T cell responses. This study evaluated the role of B cells in the induction of protective T cell immunity to a Friend murine leukemia virus (F-MuLV)-induced leukemia (FBL). B cell-deficient mice exhibited significantly reduced tumor-specific CD4+ helper and CD8+ cytotoxic T cell responses after priming with FBL or a recombinant vaccinia virus containing F-MuLV antigens. Moreover, these mice had diminished T cell responses to the vaccinia viral antigens. Tumor-primed T cells transferred into B cell-deficient mice effectively eradicated disseminated FBL. Thus, B cells appear necessary for efficient priming but not expression of tumor and viral T cell immunity.

Immunoglobulin-specific T-B cell interaction. IV. B cell presentation of idiotypic determinant(s) of monoclonal anti-surface immunoglobulin antibody to idiotope-recognizing helper T clones

Previously, we have demonstrated T-B cell interactions mediated by T cell recognition of immunoglobulin (Ig) peptide/major histocompatibility complex (MHC) class II complexes derived by the B cell processing of endogenously synthesized Ig molecules. In this report Ig-specific T-B cell interaction mediated by B cell presentation of idiotopes (Id) of anti-sIg antibodies to Id-specific T cell clones has been studied in Ig kappa-1-congenic rat strains. A panel of August (RT-1c; Ig kappa-1a) rat T helper clones specific for Id of syngeneic anti-Ig kappa-1b C3B9 monoclonal antibody (mAb) has been developed to study IdC3B9 presentation by Ig kappa-1b-bearing B cells from congenic August.1b (RT-1c; Ig kappa-1b) rats. Five of seven IdC3B9-specific T clones responded even at very low concentrations (100-200 pg/ml) of C3B9 mAb presented by Ig kappa-1b+ B cells. In contrast, the presentation of intact C3B9 mAb by nonspecific antigen-presenting cells (macrophages, Ig kappa-1a+ B cells, etc.) to IdC3B9-specific T cells was of low efficiency. The IdC3B9-specific T cell response to idiotopes of anti-Ig kappa-1b C3B9 mAb was found to be restricted by RT-1B molecule and required the processing of intact C3B9 molecule. IdC3B9 epitope recognized by C31 and C5 clones was mapped to the heavy chain of C3B9 mAb. Thus, B cells are able to present peptides related to the V region of anti-sIg Ab, i.e. Id peptide/MHC class II complexes, to Id-recognizing T cells. IdC3B9-presenting B cells are specifically activated both to proliferation and Ig production upon interaction with IdC3B9-specific T clones. Based on the results of our studies on B cell presentation of Ig epitopes to T cells a hypothetical model of Ig peptide-driven T-B cell interaction has been proposed.

Control of primary and secondary antibody responses by cytotoxic T lymphocytes specific for a soluble antigen

BALB/c mice immunized with keyhole limpet hemocyanin (KLH) develop Thy-1+ and CD8+, KLH-specific cytotoxic T cells (Tc). Such Tc cells can lyse TNP-specific B cells activated with TNP-KLH, but not with TNP-ovalbumin. Cytotoxicity was inhibited by anti-H-2K/D antibodies, but not by anti-Ia antibodies, suggesting a major histocompatibility complex class I-restricted killing. Selective enrichment for virgin and memory TNP-specific B cells revealed that the latter cells were relatively resistant to lysis by KLH-specific Tc cells. Depletion of CD8+ T cells from cultures of TNP-specific virgin B cells activated with TNP-KLH and KLH-primed T cells, increased the titer of anti-TNP antibodies in the culture supernatants. This increase was reduced if KLH-primed CD8+ T cells were added to the culture 1 day before its termination. Anti-TNP antibody secretion by memory B cells activated in the same manner was not affected by depletion or addition of CD8+ cells. These results suggest that Tc cells are generated following immunization with a soluble antigen which may participate in the down-regulation of primary B cell responses.

Distinct populations of antigen-presenting cells are required for activation of suppressor and contrasuppressor T cells by type III pneumococcal polysaccharide

Type III pneumococcal polysaccharide (S3) coupled to spleen cells (S3-SC) has been shown to activate S3-specific Ts and Tcs in mice. Ts activation required I-J identity between carrier SC and Ts donors whereas I-A identity was required for Tcs activation. The carrier SC therefore presumably function as APC for Ts and Tcs activation by S3 since they are apparently not represented by APC present in the Ts and Tcs donors. The properties of the APC required for activation of S3-specific Ts and Tcs were determined by coupling S3 to various spleen cell subpopulations and assessing the ability of the various S3-SC populations to activate Ts and Tcs. The results indicate that Ts and Tcs are preferentially activated when S3 is presented on distinct cell types. S3-specific Ts were activated when S3 was coupled to plastic adherent cells. These cells are nonadherent to anti-Ig and nonfunctional in cyclophosphamide (Cy)-treated mice and their function is eliminated following treatment of cells with either anti-I-A or anti-I-J and C. In contrast, S3-specific Tcs were activated when S3 was coupled to anti-Ig adherent SC which bear I-A and the B cell marker J11d. These cells are functional in Cy-treated mice and their function is resistant to treatment with anti-I-J and C. Thus presentation of S3 on distinct cell types results in the preferential activation of T cells having opposing immunoregulatory function.

The antiviral drug amantadine has a direct inhibitory effect on T-lymphocytes

We investigated the effect of the antiviral drug amantadine (AmTd) on polyclonal activation of thymic-dependent (T) and thymic-independent (B) lymphocytes from normal mice. In the present studies, T-lymphocytes are defined by their response to concanavalin A (Con A) and B-lymphocytes by their response to lipopolysaccharide (LPS). Polyclonal activator-induced lymphocyte proliferation was assessed by quantifying cellular incorporation of tritiated thymidine. The results show that, in a dose-dependent manner, AmTd exhibits at least 2-fold greater inhibitory activity against Con A-responding T-cells than against LPS-responding B-cells. Further, several findings demonstrate that AmTd has a direct inhibitory effect on T-lymphocytes. First, AmTd pulse treatment of isolated T-cells, but not accessory cells, abolished the T-cell response to Con A. Second, AmTd pulse treatment of the cytotoxic T-lymphocyte line, CTLL-2, markedly reduced their ability to undergo IL-2-induced proliferation. Third, proliferation of T-cells which had already undergone activation by ConA was inhibited by AmTd. Further, the finding that addition of IL-1, IL-2 or both to cultures failed to reverse inhibition of the response to ConA argues that AmTd did not interfere with endogenous production of these lymphokines. Possible implications of these findings are discussed.

Antigen presentation by B cells and macrophages of cytochrome c and its antigenic fragment when conjugated to the surface of liposomes

An in vitro antigen presentation system was used to study how antigens coupled to the surface of phospholipid vesicles (liposomes) are presented to antigen specific T cells. Liposome-bound pigeon cytochrome c (PCC) was 30-40-fold more potent than free PCC when peritoneal macrophages were the presenting cell. B cells presented surface-bound PCC, albeit less efficiently than unmodified PCC. Surface-bound peptide epitope was presented by both cell types, but not as efficiently as unmodified peptide. With the T cell epitope, antigen processing was not required since glutaraldehyde fixed cells could present surface-bound peptide.

Production of large numbers of hybridomas producing monoclonal antibodies against rat IgE using mast cell-deficient w/wv and sl/sld strains of mice

A number of different mouse strains and immunization protocols were used to attempt to make monoclonal antibodies against rat IgE for use in studies of the structure, biological activities and regulation of this class of antibody. Successful production of large numbers of monoclonal antibodies was achieved when mast cell deficient (w/wv and sl/sld) but not conventional (BALB/c, CAF1 or SJL) mice were used. These results suggest that the poor response of conventional strains of mice to rat IgE may be due to the presence of mast cells bearing high affinity receptors for IgE in these mice.

Polyclonal B-cell activation in periodontitis

The evidence that periodontitis-associated bacteria contain potent PBA factors is very strong. Clearly, antibodies directed against non-oral antigens are produced in the inflamed periodontal lesion, and PBA appears to contribute to that production. It is also clear that B cells and plasma cells are the major cell types in the periodontal lesion. Furthermore, alterations in the regulation of B-cell responses to PBA factors are associated with severe periodontal disease. However, evidence demonstrating that activated B cells and plasma cells are directly involved in the pathogenic mechanisms leading to destruction of the periodontal support is still circumstantial. Polyclonal B-cell activation and potential pathways by which PBA-stimulated cells could be involved in periodontal destruction remain largely hypothetical. It appears that IL-1 is an important osteoclast-activating agent, and that LPS, which is a potent PBA factor in many systems, can elicit IL-1 production by B cells as well as by the monocyte/macrophage lineage. Recent data indicating that IL-1 is produced by numerous malignant B-cell lines lend support for the idea that B-cell IL-1 could be important in bone resorption. It is also likely that polyclonal activation may lead to production of autoantibody such as anti-type I and anti-type III collagens, and the destruction of self tissues through ADCC reactions, immune complex formation, and complement activation. Further research is needed to determine how the B cell/plasma cell may participate in tissue injury in periodontitis, and how the B-cell response to PBA factors is regulated.

Experimental autoimmune thyroiditis in mice chronically treated from birth with anti-IgM antibodies

The role of T lymphocytes in the pathogenesis of experimental autoimmune thyroiditis in mice is well established while the role of B lymphocytes is unclear. Mice with thyroid lesions have thyroglobulin antibodies whereas these antibodies can occur in mice immunized with Tg that do not develop thyroid lesions. To determine whether thyroglobulin antibodies are necessary for the development of the thyroid infiltrates with mononuclear cells, which are characteristic for experimental autoimmune thyroiditis, AKR mice chronically treated from birth with goat anti-mouse IgM antibodies were immunized with mouse thyroglobulin in Freund's complete adjuvant when they were 7 weeks old. Control mice, similarly immunized, were chronically injected from birth with normal goat gamma-globulin. Three weeks after immunization, all mice were sacrificed, thyroglobulin antibodies in the serum were measured by hemagglutination assay and enzyme-linked immunosorbent assay, and thyroid pathology was assessed. The serum concentration of IgG and IgM, the percentage of B and T lymphocytes in the spleen (flow cytometry), and the in vitro proliferative response of spleen lymphocytes to stimulation by PHA, LPS, and Tg were also measured. All mice treated with anti-IgM antibodies did not have detectable thyroglobulin antibodies but 63% of these mice and 88% of control mice (all of which had thyroglobulin antibodies) had thyroid lesions. Mice treated with anti-IgM antibodies that did not have thyroid lesions had a more pronounced depression of B lymphocytes than similarly treated mice that had thyroid lesions. These experiments suggest that thyroglobulin antibodies are not necessary for the development of thyroid infiltrates with mononuclear cells. B lymphocytes could still participate in the production of experimental autoimmune thyroiditis by presenting thyroglobulin to helper T lymphocytes.

Presentation of antigen to T lymphocytes by non-immune B-cell hybridoma clones: evidence for specific and non-specific presentation

Non-immune SJL (H-2s) spleen cells were fused with non-secreting, non-antigen presenting (H-2d) Balb/c 653-myeloma cells and the hybridomas were cloned by two limiting dilutions. The resulting hybrid B-cell clones were tested for their antigen presentation capability to SJL T-cell lines that were specific for either lysozyme or myoglobin. In proliferative assays, 53% of the antigen presenting B-cell clones presented both myoglobin and lysozyme (general presenters) while the other 47% presented specifically either myoglobin or lysozyme (specific presenters). The ability to selectively present either myoglobin or lysozyme indicates that antigen presentation at the clonal level can be specific or non-specific depending on the particular B-cell clone.

Activated B cells can deliver help for the in vitro generation of antiviral cytotoxic T cells

The experiments described in this paper show that activated B cells can deliver help for antiviral cytotoxic T (Tc) cell responses in vitro. This conclusion is based on four observations. (i) Influenza viruses induced secondary Tc cell responses in vitro in the absence of CD4+ T cells. This capacity correlated with the B-cell mitogenicity of these viruses. (ii) Depletion of both CD4+ T cells and B cells prevented the generation of anti-influenza Tc cell responses, whereas depletion of either CD4+ T cells or B cells alone failed to do so. In addition, supplementation of unprimed B cells restored the Tc cell responsiveness of primed splenocytes that had been depleted of both CD4+ T cells and B cells. (iii) Contact between T and B cells was not obligatory for the delivery of B-cell helper signal, and hence help was mediated by a soluble factor(s). (iv) Lipopolysaccharide-activated B cells could replace the CD4+ T-cell requirement in the induction of Tc cell responses to nonmitogenic influenza virus in vitro.

B-lymphoma cells process and present their endogenous immunoglobulin to major histocompatibility complex-restricted T cells

Antigen-presenting B-lymphoma cells were transfected with the gene encoding the immunoglobulin lambda 2 light chain of MOPC315 cells (lambda 2(315). The lambda 2 chain is expressed on the cell surface of the transfectants together with the endogenous heavy chain. The transfectants present an idiotope of the lambda 2(315) light chain to class II-restricted T-cell clones. Recognition by the T cells requires processing of the lambda 2(315) light chain. From these data we conclude that B-lymphoma cells constitutively process and present their immunoglobulins. Secretion and reuptake of the light chain was not necessary for the presentation. Thus, B cells bear two types of idiotypes on their membrane, a native form as surface immunoglobulin and a processed form in the context of products of the major histocompatibility complex.

Antigen-presenting cells for unprimed T cells

The triggering requirements of T cells differ for primed and unprimed cells: primed T cells can be triggered to produce lymphokines without viable antigen-presenting cells (APCs), apparently by crosslinking the T-cell receptor (TCR). Unprimed T cells do, however, require viable APCs and here Jonathan Sprent and Mary Schaefer review what type of cells can carry out this function, with particular reference to APCs for unprimed CD8+ cells.

Murine CD4+ T cell subsets defined

We have used two monoclonal anti-murine T cell autoantibodies (SM3G11 and SM6C10) and multi-color immunofluorescence staining to resolve splenic CD4+ cells into four populations. Two of these populations (Fr. I and Fr. III, 35% and 10% of CD4+ cells) show mutually exclusive expression of these determinants and exhibit distinct functions. Fr. III secretes IL-4, but not IL-2 when activated by Con A, and includes memory T cells responsible for secondary antibody formation. In contrast, Fr. I secretes IL-2 but not IL-4 in response to Con A, and does not contribute to the secondary antibody response. Furthermore, these two fractions exhibit differential accessory cell dependence. Whereas Fr. III responds with B cells (and also non-B cells) as accessory cells in Con A-induced activation, Fr. I requires non-B cells. However, we found that many CD4+ cells (Fr. II, 40% of CD4+ cells) express both determinants and are not distinguishable with regard to lymphokine secretion, accessory cell effect, and memory T cell activity. Curiously, the fraction expressing neither determinant (Fr. IV, 10% of CD4+ cells) is unresponsive to experimental conditions used here. We discuss the possible relationships between these T cell subsets and the implications of differential expression of these determinants.

Presentation of antigen to T lymphocytes by non-immune B-cell hybridoma clones: evidence for specific and non-specific presentation

Non-immune SJL (H-2s) spleen cells were fused with (H-2d) Balb/c 653-myeloma cells and the hybridomas were cloned by two limiting dilutions. The resulting hybrid B- cell clones were tested for their antigen presentation capability to SJL T-cell lines that were specific for either lysozyme or myoglobin. In proliferative assays, 53% of the antigen presenting B-cell clones were able to present both myoglobin and lysozyme (general presenters) while the other 47% presented specifically either myoglobin or lysozyme (specific presenters). The ability to selectively present either myoglobin or lysozyme indicates that antigen presentation at the clonal level can be specific or non-specific depending on the particular B-cell clone.

Can B cells turn on virgin T cells?

The first event in the initiation of an immune response is the capture and presentation of antigen to T cells. Such presentation involves two distinct steps: (1) display of the antigen, which requires uptake, processing and re-expression of the antigen in association with MHC molecules on the presenting cell surface; and (2) triggering, in which the presenting cell provides signals leading to the activation of the responding T cell. Two sorts of cells can capture antigens, the 'professional' antigen-presenting cells (APCs) such as dendritic cells and macrophages, and the B cells. Both types of cells can display antigens and the APCs are known to be able to trigger resting T cells. But despite in vitro evidence that certain B-cell types can reactivate previously-activated T cells, it is not yet clear whether a B cell can initiate an immune response by providing the signals necessary to activate a resting T cell. We reasoned that resting B cells should not have this capacity because of the problems this would present with tolerance to self idiotypes. By exploiting the unique properties of the avian haematopoietic system, we have examined the presenting capacity of B cells in vivo and found that resting B cells are indeed unable to activate resting T cells.

Role for intracellular proteases in the processing and transport of class II HLA antigens

Human B-lymphoblastoid cell lines (B-LCL) incubated with the protease inhibitor leupeptin accumulate complexes of class II HLA antigens with a series of Mr 21,000-23,000 basic proteins termed leupeptin-induced proteins (LIP). The appearance of class II antigen-associated LIP coincides with the disappearance of class II antigen-associated invariant (I) chain. Glycopeptides generated by in vitro proteolysis of LIP and I chain using Staphylococcus aureus V8 protease are identical as determined by electrophoresis in sodium dodecyl sulfate. These results suggest that LIP is a proteolytic product derived from the I chain and are consistent with the view that further in vivo proteolysis of LIP by a leupeptin-sensitive enzyme normally facilitates its release from class II antigens. Incubation of B-LCL with monensin, which traps class II antigens and associated I chain in the Golgi apparatus, or chloroquine, which neutralizes intracellular acidic compartments and inhibits I-chain dissociation, blocks the leupeptin-induced appearance of LIP. Treatment of LIP with endoglycosidases F and H shows that both of its N-linked oligosaccharides are in the complex form, indicating that proteolysis of class II antigen-associated I chain to generate LIP occurs in a late-Golgi or post-Golgi compartment. The compartment in which these proteolytic events occur may be identical to the site in macrophages and B lymphocytes where foreign antigens are processed and interact with class II HLA molecules.

Involvement of antigen and I-J determinants in the induction of effector T suppressor cells by immune B cells

Immune B cells induce effector T suppressor cells in vitro. The B cells act as antigen-presenting cells, and express both I-A and I-J determinants. Antigen and I-J determinants are required for the induction of suppressor T cells by immune B cells, but I-A determinants are not. These findings indicate that precursors of suppressor T cells appear to recognize antigen in the context of I-J determinants on the surface of immune B cells.

Evolutionary and somatic selection of the antibody repertoire in the mouse

The repertoire of antibody variable (V) regions has been subject to evolutionary selection, affecting both the diversity of V region genes in the germline and their expression in the B lymphocyte population and its subsets. In ontogeny, contact with an antigen leads to the expansion of B cells expressing antibodies complementary to it. In a defined phase of B cell differentiation, new sets of V regions are generated from the existing repertoire through somatic hypermutation. Cells carrying advantageous antibody mutants are selected into the memory compartment and produce a stable secondary response upon reexposure to the antigen.

Antigen uptake and accumulation in antigen-specific B cells

Using EBV-transformed antigen-specific B-cell clones as APC, we have started to analyze the factors that govern the uptake of antigen by B cells and lead to its efficient presentation to T cells. The binding of antigen to SIg can be blocked by soluble antibodies that react with the same epitope that is seen by the B cell's SIg, but not by antibodies that bind to different epitopes. By studying the antigen presenting capacity of B cells that had been pulsed with antigen in different conditions, we came to the conclusion that specific B cells work as vacuum cleaners, i.e. over time they accumulate the antigen that binds to SIg. This effect results from the difference between the rate of influx (approximately 5-10 times the amount of antigen bound is internalized every hour by receptor-mediated endocytosis) and the rate of loss (the half-life of processed antigen relevant for T-cell activation is about 1 d). T cells specific for mouse Ig are triggered much more efficiently by mouse anti-human Ig than by mouse antibodies directed against other B-cell surface antigens, suggesting that SIg are extremely efficient for antigen internalization and processing.