Biomedical subjects

E S Vitetta

Publications and source records attributed to E S Vitetta.

At least 163 records · Page 9Linked to original sources

PubMed1987

Activation of murine B cells from different tissues with different mitogens. II. Isotype distribution of secreted immunoglobulins in the presence and absence of IL-4-containing T cell supernatants.

In the accompanying report, we have compared a B cell-specific protein mitogen, Salmonella thyphimurium mitogen (STM), to lipopolysaccharide (LPS), dextran sulfate (DxS), and the combination of LPS/DxS with regard to their ability to induce B cell proliferation and differentiation. The results of these studies demonstrated that STM, LPS, and LPS/DxS induce significant expression of cytoplasmic immunoglobulin (Ig). In this report, we have analyzed the distribution of isotypes induced by each of these mitogens in the presence or absence of interleukin-4 (IL-4) containing T cell supernatants (SN), since IL-4 increases the secretion of IgG1 and IgE and decreases the secretion of IgG3 and IgG2b in LPS-stimulated splenic B cells. These experiments were designed to determine whether LPS was unique in its ability to "program" B cells to respond to IL-4 by secreting IgG1, as has been suggested in our previous studies. The current experiments also addressed the issue of whether splenic B cells were unique in their response to IL-4 by using B cells from other tissues. The efficiency of induction of cytoplasmic Ig measured in the preceding report was STM greater than LPS/DxS greater than LPS. DxS did not induce a significant level of cell proliferation, cytoplasmic Ig, or secreted IgM and inhibited the LPS-induced IgM response by approximately 50%. In contrast, STM induced an extremely high level of IgM secretion in splenic B cells. In this report, we demonstrate that the addition of IL-4-containing T cell SN to splenic B cells stimulated with LPS, LPS/DxS or STM increased the level of IgG1 and reduced the level of IgM, IgG3, and IgG2b secretion.(ABSTRACT TRUNCATED AT 250 WORDS)

Animals↗

PubMed1986

A simplified microassay for inhibition of protein synthesis in reticulocyte lysates by immunotoxins.

We describe a simplified cell free reticulocyte lysate protein synthesis assay which has been shown to reliably demonstrate dose dependent inhibition of protein synthesis with cycloheximide and with four ricin A chain immunotoxin conjugates. Maximal [3H]leucine incorporation was observed using 40 microliter of reticulocyte lysate per microtiter well and by incubating samples for 30 minutes before harvesting with a multiple automated sample harvester. Trichloroacetic acid precipitation and hydrogen peroxide decolorization were not required with this method. This rapid, simplified assay system will facilitate testing of agents that inhibit protein synthesis, particularly immunotoxins.

Animals↗

PubMed1986

Recombinant IL 2 but not recombinant interferon-gamma stimulates both proliferation and IgM secretion in a Ly-1+ clone of neoplastic murine B cells (BCL1).

We have used a lymphokine-responsive clone (3B3) of B leukemia cells (BCL1) to examine the effects of several recombinant and purified lymphokines. Cells from BCL1-3B3 were induced to secrete IgM in the presence of recombinant interleukin 2 (rIL 2) (10 to 50 U/ml); a concomitant increase in proliferation was observed. Recombinant interferon-gamma (rIFN-gamma) was a potent inhibitor of proliferation. In addition, rIFN-gamma did not induce an increase in IgM secretion and, when added to rIL 2-stimulated BCL1-3B3 cells, completely blocked IgM secretion at a concentration of 10 U/ml. Purified and recombinant IL 1 (rIL 1) had no significant effect on differentiation either alone or in combination with rIL 2 and/or rIFN-gamma. However, rIL 1 was able to synergize with rIL 2 in enhancing the proliferation of BCL1-3B3. The ability of cells to respond to rIL 2 was limited to the in vitro (Ly-1+) clones of BCL1 cells since the in vivo derived (Ly-1-) BCL1 cells did not differentiate in response to IL 2. Consistent with their functional response to rIL 2, cells from the in vitro clone (3B3) are IL 2-receptor-positive (IL-2R+) and the in vivo derived BCL1 cells are IL-2R-. A second set of neoplastic B cell clones derived from the AKR 225 lymphoma did not respond to rIL 2 even though they expressed receptors for IL 2 and could be induced by T cell supernatant to secrete IgM, thus indicating that expression of IL 2R is not the sole requirement for IL 2 responsiveness. The monoclonal anti-IL 2R antibody (7D4) mimicked IL 2 in its ability to stimulate differentiation of BCL1-3B3 cells. These data suggest that rIL 2 and the monoclonal anti-IL-2R antibody are capable of inducing a differentiative response in the Ly-1+ BCL1-3B3 cells that is functionally equivalent to the response evoked by the previously described lymphokine B cell differentiation factor for IgM (BCDF mu). Thus, two distinct lymphokines appear to be providing a similar signal to a clonal neoplastic B cell population. Furthermore, rIL 2 is capable of providing both a proliferative and a differentiative signal.

Animals↗

PubMed1986

Characterization of the physical interaction between antigen-specific B and T cells.

It has been assumed that physical interaction between B cells and helper T cells in the presence of specific antigen is an early and essential step in the physiologic antibody response to thymus-dependent antigens. The present studies were designed to examine this physical interaction by employing carrier-specific T hybridoma cells that can provide help to highly enriched hapten-binding B cells. Direct conjugation of the B and T cells can be visualized at both the light and electron microscopic level and the number of conjugates can be directly quantified. Before their effective conjugation with T cells, the B cells must be incubated with specific antigen for 4 to 6 hr. After this time, the T cells form conjugates with the B cells within 5 min. Conjugate formation requires hapten specificity, carrier specificity, covalent linkage between hapten and carrier, and is MHC restricted. Two types of T-B conjugates were observed by electron microscopy: an antigen-independent attachment of B cell microvilli to small portions of the T cell surface and an antigen-dependent, intimate apposition of large areas of the plasma membranes of the T and B cells. The kinetics of development of the two modes of interaction suggest that the second type may be important for signal transduction, since the number of T and B cells showing intimate interactions increases with time. Monoclonal antibodies directed against Thy-1.2, LFA-1 alpha, L3T4, and I-A partially block conjugation of the two cell types, suggesting that these surface molecules are involved in T-B interaction.

Animals↗

PubMed1986

Evaluation of ricin A-chain immunotoxins directed against human T cells.

We have synthesized four immunotoxins (ITs) by covalently coupling the A chain of ricin to murine monoclonal antibodies that recognize surface antigens on human T cells. Treatment of human peripheral blood lymphocytes with either 10.2-A, directed against the CD5 (Tp67) antigen, or 64.1-A, directed against the CD3 (Tp19) antigen, abolished protein synthesis in cells subsequently cultured with phytohemagglutinin (PHA). In contrast, two other ITs (9.6-A and 35.1-A), both directed against the CD2 (Tp50) antigen, had minimal effects on protein synthesis in PHA-stimulated cells. The binding of each IT to T cells was shown by immunofluorescence with fluorescein-conjugated goat anti-mouse immunoglobulin (FITC-GAMIg) and fluorescein-conjugated rabbit anti-ricin A-chain (FITC-RAR) antibodies. Activity of the ricin A chain in each IT was demonstrated by its ability to inhibit protein synthesis in a cell-free reticulocyte lysate assay. Ultrastructural immunoperoxidase analysis of IT internalization showed that ineffective and effective ITs were endocytosed at the same rate (50% of cells had labeled endosomes after 15 min). However, ineffective IT 35.1-A was more rapidly delivered to lysosomes (15-30 min) than effective ITs (10.2-A and 64.1-A) (greater than or equal to 30 min). The data support the hypothesis that there are several distinct pathways for internalization of ITs and that the ability of ricin A chain to reach and inactivate ribosomes may depend upon the specific membrane receptor involved in binding a given IT, its route of internalization, and the rate of entry of the IT into lysosomes.

Antibodies, Monoclonal↗

PubMed1986

A monoclonal anti-mouse LFA-1 alpha antibody mimics the biological effects of B cell stimulatory factor-1 (BSF-1).

This study describes the generation of a monoclonal mouse x rat antibody (G-48) that recognizes a determinant on the serologically defined LFA-1 alpha-chain. It immunoprecipitates two noncovalently associated polypeptides of 176,000 and 95,000 Mr respectively from lysates of radioiodinated BCL1 cells, T cells, and B cells. G-48 mimics the biological actions of BSF-1 by inducing increased levels of Ia antigen expression on resting B cells, augmenting the proliferation of anti-delta-stimulated B cells, and in insolubilized form, inducing IgG1 secretion by LPS-activated B cells. G-48 does not have BCDF mu, BCGF II, nor IL 2 activity. These results demonstrate that LFA-1 plays an important role in B cell activation, proliferation, and differentiation.

Animals↗

PubMed1986

Regulation of the expression of multiple class II genes in murine B cells by B cell stimulatory factor-1 (BSF-1).

We have studied the mechanisms governing the induction of class II major histocompatibility complex (MHC) antigens on murine B lymphocytes by B cell stimulatory factor-1 (BSF-1). BSF-1 induced a dramatic (10- to 15-fold) and selective increase in class II MHC antigen expression, as judged by flow cytometry. Analysis of radiolabeled membrane proteins on BSF-1-induced B cells also showed that the expression of class II MHC antigens was increased 10- to 15-fold. Biosynthetic labeling studies established that a selective increase in the translation of class II MHC molecules could be detected at 5 to 6 hr post-addition of BSF-1 to culture. After 16 hr of culture, when cell surface expression was induced 12- to 15-fold, biosynthesis rates of class II MHC antigens was induced fivefold to sixfold. The biosynthesis of the I region-associated invariant chain (Ii) was also enhanced. Actinomycin D abrogated the increased biosynthetic labeling of class II products, suggesting transcriptional regulation of expression. At 6 hr after addition of BSF-1, there was a twofold increase in the steady-stage level of class II mRNA, which was slightly increased at 16 hr. The BSF-1-induced increase in surface Ia antigen is dependent on new transcription and translation. However, posttranslational events that lead to a decrease in membrane antigen turnover rate or to an increase in protein stability probably also play a significant role in the hyperexpression of class II antigens on BSF-1-treated B lymphocytes.

Animals↗

PubMed1986

Isolation of antigen-binding virgin and memory B cells.

One major obstacle in studying the activation of antigen-specific B cells is the small number of B cells reactive with a particular antigenic epitope. In this report, we describe a method by which large numbers of highly purified antigen-binding cells can be obtained. We have shown that by varying the haptenation level of the erythrocytes used for rosetting, we can purify antigen-binding B cells which have different affinities for the antigenic epitope. Thus, memory cells (which have receptors of higher affinity) can be prepared and these cells are essentially free of contaminating virgin cells. The effects of varying the haptenation levels on the red cells used for purifying the B cells can, in turn, be related to the precursor frequency of secreting cells following their activation with T cells and antigen.

Animals↗

PubMed1986

Growth inhibition of a B cell leukemia: evidence implicating an anti-idiotype immune response for protective tumor immunity.

BCL1, a spontaneous surface IgM (mu lambda)-positive (sIgM+) B cell leukemia of BALB/c (Igha) origin rarely grows in the Ig heavy chain (Igh) congenic mouse C.B-20 (Ighb) but is highly metastatic and lethal in the host strain of origin. Previous studies indicated that BCL1 tumor immunity in C.B-20 mice was associated with a T cell-mediated immune response against H-40, a minor histocompatibility (H) antigen controlled by a gene linked to the Igh locus. However, we observed that BCL1 leukemia grew progressively in BAB-14 (Igha/b) mice, a strain capable of generating an anti-H-40 immune response. This suggested that anti-H-40 immunity was insufficient for protection and implied that an Igh-V (variable) region gene product was also important for BCL1 growth inhibition. We therefore evaluated the role of two possible Igh-V region-linked gene products in BCL1 growth inhibition; namely, an Igh-V region-linked minor H antigen or alternatively the BCL1 IgM idiotype (Id). We could find no evidence for an Igh-V region-linked minor H antigen because immunosuppressed (500 R) CB-20 mice reconstituted with C.B-20 anti-BAB-14 splenocytes were susceptible to BCL1 growth, whereas recipients reconstituted with C.B-20 anti-BALB/c splenocytes were resistant to BCL1 challenge. In contrast, C.B-20 mice immunized against purified BCL1 IgM protein could adoptively confer BCL1 tumor immunity. C.B-20 mice immunized against other BALB/c IgM myeloma proteins containing either lambda or kappa light chains failed to protect C.B-20 mice suggesting that recognition of a unique determinant (Id) and not an allotype was crucial for tumor immunity. The BCL1 mu-chain appeared to make the major contribution to the idiotypic determinant because a hybridoma product composed of BCL1 mu-chains and BALB/c kappa-chains still elicited BCL1 immunity. Adoptive transfer of C.B-20 anti-BCL1 Id splenocytes into irradiated recipients that prevented an anti-H-40 response due to H-40 tissue expression failed to adoptively confer BCL1 immunity. Thus, these data suggest that BCL1 growth inhibition requires a T cell-mediated response against both H-40 and the BCL1 Id; these responses must be elicited concurrently in the tumor-bearing host to achieve protective BCL1 immunity.

Animals↗

PubMed1986

Tumor dormancy. I. Regression of BCL1 tumor and induction of a dormant tumor state in mice chimeric at the major histocompatibility complex.

The growth of the BCL1 tumor in murine H-2 chimeras was studied. Lethally x-irradiated BALB/c mice were reconstituted with C57BL/6 bone marrow that had been depleted of T cells. When chimerism was established 90 to 120 days later, large doses of BCL1 cells were injected. The tumor grew progressively, reaching a peak level of as many as 10(9) tumor cells per animal by 40 days after inoculation. After that time, the tumor regressed in all the chimeric animals, and by 100 days after inoculation, virtually all the animals appeared disease free as judged by an absence of BCL1-idiotype-positive cells in the spleen and peripheral blood, a normal spleen size, and absence of an elevated white blood cell count. Such animals were followed for as long as 8 mo after tumor inoculation and remained disease free. However, transfer of graded numbers of splenocytes from these animals into normal BALB/c recipients resulted in development of tumor in recipients receiving 100 or more spleen cells. These results indicate a large tumor burden in the spleen of each donor, namely, 10(6) to 10(7) BCL1 cells. The present model should facilitate characterization of the mechanisms underlying tumor dormancy.

Animals↗

PubMed1986

Double isotype production by a neoplastic B cell line. I. Cellular and biochemical characterization of a variant of BCL1 that expresses and secretes both IgM and IgG1.

We have subcloned the in vitro-adapted murine B cell leukemia, BCL1.B1, to obtain a variant that expresses both IgM and IgG1. By fluorescence analysis, radioiodination, and immunoprecipitation of cell surface Ig, and by RIA of medium from limiting dilution cultures, we have shown that: (a) all the cells express and secrete both isotypes. The heavy chains of both IgG1 and IgM have the apparent molecular weights of membrane mu and gamma 1 chains; (b) both isotypes bear the same idiotype as determined by immunoprecipitation with antiidiotypic antibody, and both use the same VDJ rearrangement as shown by Southern blotting; and (c) the cells express the membrane and secreted forms of mRNA for both mu and gamma 1 but not gamma 2b or gamma 3. Taken together, the data suggest that all the cells are synthesizing, expressing on their surface, and secreting two isotypes that use the same VDJ rearrangement in the DNA and express the same serologically-defined idiotype. The molecular basis responsible for the production of the two isotypes in a single cell is the subject of the accompanying paper.

Animals↗

PubMed1986

Double isotype production by a neoplastic B cell line. II. Allelically excluded production of mu and gamma 1 heavy chains without CH gene rearrangement.

In our accompanying paper, we described a switch variant (BCL1.2.58) that expresses membrane and secreted forms of IgM and IgG1. Both IgM and IgG1 share the same idiotype and use the same VDJ rearrangement. Here, a detailed Southern blot analysis of the entire constant region of the Ig heavy chain (Ig CH) locus of parental (BCL1.B1) and variants (BCL1.B2) DNA showed no detectable rearrangement. Similar analysis of the JH-C mu region led to the conclusion that two heavy chain alleles present in the IgM/IgG1-producing variants carried the same VDJ rearrangement but differed in their 3' flanking regions. One chromosome 12 did not carry any Ig CH genes, whereas, the other chromosome 12 carried one copy of CH genes. In BCL1.B1, however, each of the chromosome 12 alleles carried a full copy of CH genes. Karyotypic analysis confirmed the presence of two translocated t(12;16) chromosomes in both BCL1.2.58 and BCL1.B1 cells, with a break 5' to the VH locus at the distal region (12F2) of chromosome 12, and at the proximal region below the centromere (16B3) of chromosome 16. We conclude that double production of IgM and IgG1 in BCL1.B2 is accomplished by transcription of the corresponding CH genes in germline configuration using a single VDJ on the same chromosome 12.

Alleles↗

PubMed1986

B cell-stimulatory factor 1 (BSF-1) promotes growth of helper T cell lines.

T cell-derived supernatants (SN) that contain B cell-stimulatory factor 1 (BSF-1) and lack IL-2 promote the growth of the IL-2-dependent T cell line, HT-2, as well as three other clones or lines of T cells that can provide help to B cells. The BSF-1 purified from these SNs promotes growth of HT-2 cells approximately 50% as effectively as purified IL-2. A potential involvement for contaminating IL-2 in the BSF-1 preparations was excluded by the demonstration that anti-BSF-1 mAbs blocked the BSF-1-induced growth of HT-2 cells; in contrast, these antibodies did not block the IL-2-induced proliferation of the HT-2 cells. In addition, anti-IL-2 mAbs or anti-IL-2-R antibodies blocked the HT-2 growth-promoting activity of purified IL-2, but not BSF-1. Finally, BSF-1 promoted only a very modest growth of Con A-induced T cell blasts, and failed to induce significant growth in seven other cytotoxic, alloreactive, and long-term T cell lines. Taken together, these results indicate that in addition to its known effects on resting and LPS-stimulated B cells, BSF-1 can promote growth of certain subsets of activated T cells, in particular, those that provide help to B cells.

Animals↗

PubMed1986

Changes in phospholipid metabolism during B lymphocyte activation.

We have examined phospholipid metabolism in murine B lymphocytes stimulated with anti-Ig bound to Sepharose. T cell-depleted splenic B lymphocytes cultured with Sepharose-coupled, affinity-purified goat anti-mouse Ig (GAMIg) increased the incorporation of 32PO4 into phosphatidic acid and phosphatidylinositol within 3 hr and increased [3H]-thymidine uptake at 48 hr. No increase in labeling was observed in phosphatidylethanolamine, phosphatidylcholine, or phosphatidylserine. Based on both negative and positive selection procedures, it was demonstrated that these responses occurred in B lymphocytes. In contrast to the thymidine uptake response of the GAMIg-stimulated B lymphocytes, the phospholipid response did not require the presence of accessory cells or exogenous cytokines. The same selective changes in phospholipid metabolism were observed in neoplastic B lymphocytes (BCL1) after treatment with Sepharose anti-mu, but not with Sepharose anti-Ia or Sepharose normal Ig. The dose-response relationships of 32PO4 incorporation into phosphatidic acid and phosphatidylinositol and [3H] thymidine uptake were nearly identical in BCL1 cells. The results of these experiments indicate that interaction of B lymphocytes with insolubilized anti-Ig results in prompt and selective changes in phospholipid metabolism that appear to be correlated with B lymphocyte proliferation.

Animals↗

PubMed1986

In vitro activation of murine antigen-specific memory B cells by a T-dependent antigen.

A procedure that generates an enriched population (60 to 85%) of memory B cells specific for TNP (TNP-MABC) was employed. The activation requirements of TNP-MABC for the T-dependent antigen TNP-KLH and carrier-primed helper T (Th) cells were compared to those for TNP-binding cells from nonimmune mice (TNP-ABC). Proliferation and differentiation of TNP-MABC in response to cognate recognition of antigen requires less antigen and fewer carrier-primed Th cells than the activation of TNP-ABC. Furthermore, responses of the TNP-MABC were of a greater magnitude. Non-cognate activation induces a low level of proliferation of both TNP-ABC and TNP-MABC, but induces differentiation of TNP-MABC only. Percoll density fractionation of spleen cells prior to enrichment for TNP-MABC suggests that the small, dense cell population responds to cognate, but not to non-cognate activation. FACS separation of TNP-MABC by surface Ig isotype reveals that approximately 80% of the secondary IgG response is derived from cells expressing sIgG. Such cells constitute less than 10% of the total number of TNP-MABC. Limiting dilution studies with sorted TNP-MABC indicate that sIgG+ TNP-MABC are enriched for precursors that give rise to a large clone size. The in vitro results indicate the existence of three putative pathways for antigen-specific memory B cell activation by a T-dependent antigen: 1) sIgG+ cells differentiating into IgG-secreting cells; 2) sIgM+ cells differentiating into IgG-secreting cells; and 3) sIgM+ cells differentiating into IgM-secreting cells.

Animals↗

PubMed1986

Purification of ricin A1, A2, and B chains and characterization of their toxicity.

This paper describes a protocol for the preparation of highly purified A (A1 and A2) and B chains of the plant toxin, ricin, and biochemical and biological characterization of these proteins. Intact ricin was bound to acid-treated Sepharose 4B and was split on the column into A and B chains with 2-mercaptoethanol. The A chains were eluted with borate buffer containing 2-mercaptoethanol. A1 and A2 were then partially separated by cation exchange chromatography and the contaminating B chain was removed by affinity chromatography on Sepharose-asialofetuin and Sepharose-monoclonal anti-B chain. The B chain was eluted from the Sepharose 4B column by treatment with galactose and was further purified by cation and anion exchange chromatography; contaminating A chains were removed by affinity chromatography on Sepharose-monoclonal anti-A chain. The purified A and B chains were active as determined by their ability to inhibit protein synthesis in a cell-free assay and their binding to asialofetuin, respectively. Furthermore, by polyacrylamide gel electrophoresis, toxicity in mice, and toxicity on several different cell types, both A and B chains were shown to be minimally cross-contaminated. Finally, it was shown that ammonium chloride significantly enhanced the nonspecific toxicity of B chains for cells in vitro. In contrast, ammonium chloride did not enhance either the nonspecific toxicity of A chains in vitro or the specific toxicity of A chain-containing immunotoxins prepared with the highly purified A1, A2 chains.

Ammonium Chloride↗

PubMed1986

The effect of antibody valency and lysosomotropic amines on the synergy between ricin A chain- and ricin B chain-containing immunotoxins.

The cytotoxicity of A chain immunotoxins containing IgG or Fab fragments specific for the surface immunoglobulin of the Daudi cell line was assessed in the presence of B chain immunotoxins (IgG or Fab) or lysosomotropic amines, or both. The concentration required for 50% inhibition of protein synthesis (IC50) in Daudi cells was 1.3 X 10(-8) M for IgG-A and 5 X 10(-8) M for Fab-A. The toxicity of both A chain immunotoxins was enhanced twofold by ammonium chloride. In the presence of A chain immunotoxins and ammonium chloride, a maximum of 99 and 90% reduction of clonal precursors was obtained with IgG and Fab-A chain immunotoxins respectively. Immunotoxins containing ricin B chain and IgG or Fab fragments specific for the antibody portion of A chain immunotoxins were used as secondary "piggyback" immunotoxins to treat cells that were pretreated with A chain immunotoxins. Both B chain immunotoxins were nontoxic at 1 X 10(-6) M. When added to target cells pretreated with specific A chain immunotoxins, the IC50 of the A chain immunotoxins was decreased up to 16-fold in the absence of ammonium chloride. In contrast to the results obtained with A chain immunotoxins alone, ammonium chloride significantly increased the toxicity of the complete piggyback system, resulting in the killing of 99.999% or five logs of target cells in the clonal assay. This decreased the IC50 of A chain immunotoxins up to 116-fold when compared with A chain immunotoxin alone. This enhanced toxicity was independent of the valency of either immunotoxin.

Adjuvants, Immunologic↗

PubMed1986

Synergy between immunotoxins prepared with native ricin A chains and chemically-modified ricin B chains.

Ricin B chains treated with chloramine-T in the presence or absence of NaI show a 100-fold to 200-fold reduction in their ability to bind to the galactose-containing protein asialofetuin. Such treated B chains do not form covalently associated homodimers with treated B chains or heterodimers with native ricin A chains. Furthermore, they cannot enhance the toxicity of a ricin A chain-containing rabbit anti-human immunoglobulin (RAHIg-A) for Daudi cells. However, when such B chains are coupled to goat anti-rabbit Ig (GARIg), they potentiate the killing of RAHIg-A-treated Daudi cells only slightly less effectively than GARIg coupled to native B chains. Furthermore, if GARIg-B chain conjugates are treated with chloramine-T after coupling, they fail to bind to asialofetuin but enhance the killing of Daudi cells treated with RAHIg-A. These results demonstrate that the ability of ricin B chains to bind to galactose and to enhance the toxicity of ricin A chains (in the form of an antibody-A chain) can be operationally separated. Thus, the two functions of the B chain may reside on separate domains of the molecule.

Animals↗