A CD8 polypeptide that is lost after passing the Golgi but before reaching the cell surface: a novel sorting mechanism.

The murine CD8 T cell differentiation antigen is a glycoprotein expressed on the cell surface as a heterodimer comprising the products of two closely linked genes, Ly-2 and Ly-3. The Ly-2 gene encodes, through a mechanism of alternate splicing, two polypeptide chains, alpha and alpha', that differ from one another in the lengths of their cytoplasmic tails. All T cells transcribe and translate both the alpha and alpha' polypeptides of Ly-2 and form heterodimers of each of these polypeptides disulphide-bonded with the Ly-3 polypeptide. However, there is very specific, developmentally controlled regulation of the expression of these heterodimers on the cell surface. Namely, immature T cells show no discrimination of CD8 molecules and express on their cell surface heterodimers containing the Ly-3 polypeptide linked to either the alpha or alpha' chain of Ly-2. In contrast, mature T cells express on their cell surface predominantly the heterodimer containing the Ly-2 alpha chain, the species which has a cytoplasmic tail. Moreover, in mature T cells the complexes which contain the alpha' chain are retained within the cell late in processing. These data emphasize the importance of the CD8 accessory molecule in the development of the functional T cell repertoire and uncover a novel protein sorting mechanism in mature T cells.     

Expression of a T cell receptor-like molecule on normal and malignant murine T cells detected by rat monoclonal antibodies to nonclonotypic determinants

A T cell receptor-like molecule with a dimer structure of 45 kilodaltons (Kd) under reducing and 90 Kd under nonreducing conditions was detected on the surface of two murine T lymphoma lines, EL-4 and MBL-2, by two rat monoclonal antibodies. The two antibodies seemed to react with different determinants on the same molecule. The antibodies did not react with the surface of normal T cells as tested by flow cytometric analysis of cell surface staining. Two-dimensional gel electrophoresis (IEF vs SDS-PAGE) and tryptic peptide analysis revealed the molecule to consist of two chains with different isoelectric points and different tryptic peptides. A conventional antiserum was raised against the heterodimer purified from EL-4 cells. The immune serum did not bind to the surface of normal T cells. However, the immune serum as well as the monoclonal antibodies immunoprecipitated the dimer molecules from detergent-solubilized normal thymocytes and spleen cells. The dimer molecule was detected on both immature and mature thymocytes. These results suggest that the antibodies detect non-clonotypic determinants on a T cell receptor-like protein. The determinants are masked on the surface of normal T cells, whereas they are exposed on the surface of at least two T lymphoma cell lines. Three polypeptides of 30 Kd, 25 Kd, and 15 Kd were also coprecipitated with the heterodimer from MBL-2 cells. These proteins may associate with the heterodimer and may be masking the antigenic determinants on normal T cells. The relationship between the heterodimer molecule described here and the T cell antigen receptor or the human T cell antigen 9.3 is still unknown.     

Plasticity of T cell function. Cloned EL-4 lymphoma cells may help or suppress a primary antibody response depending on their own concentration and the assay system

Cloned EL-4 lymphoma T cells were tested in limiting dilution experiments for their capacity to suppress or to help the primary humoral immune response of spleen cells (or T cell-depleted spleen cells) to the Ag SRBC and 4-hydroxy-3-iodo-5-nitro-phenyl-keyhole limpet hemocyanin. EL-4 clones are able to suppress up to 80% of the total IgM responses in both systems, as well as to help. Suppression and help fluctuate between high and low levels with the numbers of EL-4 cells placed into tissue cultures. In Poisson plots, this is reflected as a "typical curve": usually one or two frequencies are estimated (e.g., integral of 1/4 and approximately 1/1000 for suppression and approximately 1/6 and approximately 1/200 for help), which appear regulated with increasing numbers of cells seeded. Control experiments showed that EL-4 cells need to be alive to exert the effects. EL-4 cells do not serve as additional antigen, do not induce an isotype switch and are not cytotoxic. Help and suppression are not restricted by the MHC. Help requires the presence of a small number of normal T cells in the assay system, indicating that EL-4 cells do not replace specific helper T cells. When a number of control cell lines were analyzed under identical circumstances, similar effects were observed with most long term T cell lines or clones expressing a T cell receptor, whereas cells of non-T lineages and T cells not expressing a T cell receptor did not show the phenomena. The results suggest a functional plasticity of T cells, dependent on cell numbers and the assay system used, and expressed via T cell communication.     

Characterization of SJL T cell clones responsive to syngeneic lymphoma (RCS): RCS-specific clones are stimulated by activated B cells

To gain insight into the nature of the syngeneic T cell-stimulating molecules on SJL lymphoma cells (RCS), a panel of eight Ly-1+2- T cell clones that are specific for transplantable RCS has been generated. All of these clones proliferate vigorously in response to two independent RCS lines and to LPS-activated syngeneic or F1 B cell blasts, but not to unstimulated SJL spleen cells or to allogeneic B cell blasts. Only one RCS-specific clone displays a proliferative response to (SJL X BALB/c) resting spleen cells, suggesting that I-E molecules are not the source of stimulation of RCS-responsive cells. Responses of the T cell clones to both RCS and syngeneic LPS-activated B cells are inhibited by monoclonal antibodies to I-A antigens, and not by antibody to I-E antigens. These findings suggest that RCS-responsive T cells are stimulated either by syngeneic I-As alone, in a form expressed on activated B cells, or by I-As in combination with X, where X is a cell surface antigen present on B cells at certain stages of differentiation.     

CD73 and Ly-6A/E distinguish in vivo primed but uncommitted mouse CD4 T cells from type 1 or type 2 effector cells

Primed CD4 T cells may develop into effector T cells such as Th1 and Th2, or remain uncommitted as Th primed precursor (Thpp) cells that can subsequently differentiate into Th1 and Th2 cells. Although mouse Thpp-like cells have also been identified among spleen and particularly lymph node cells, further characterization of these cells has been difficult without a defining cell surface marker. Using Affymetrix GeneChips followed by FACS analysis, we found that in vitro-derived Thpp cells expressed CD73 but not Ly-6A/E, whereas Th1 and Th2 cells showed the reciprocal pattern. CD73+ Ly6A/E- memory CD4 T cells were identified in normal C57BL/6 mice, and the proportion of these cells was highest in lymph nodes, lower in spleens, and lowest in the lungs. These cells produced IL-2 and MIP-1alpha, but much less IL-4 and IFN-gamma than CD73- Ly6A/E+ cells. Similar results were obtained with additional Ly-6.2 mouse strains, but not Ly-6.1 strains. Restimulation of Thpp-like CD73+ Ly-6A/E- cells in Th1- or Th2-polarizing conditions induced differentiation into populations producing mainly IFN-gamma or mainly IL-4, respectively. In contrast, the effector-like CD73- Ly-6A/E+ population was more committed, and continued to produce both IL-4 and IFN-gamma in both conditions. CD73 and Ly-6A/E expression therefore identify a population of Thpp-like cells in C57BL/6 mice and at least some other Ly-6.2 mice.     

Ly-6.2: A new lymphocyte specificity of peripheral T-cells

A new cell-membrane alloantigen determining locus, Ly-6, has recently been described, and the single specificity Ly-6.2 has been defined by the serum (BALB/c× A)F₁ anti-CXBD. Using both fluorescence and cytotoxicity, we found this specificity predominantly on peripheral (extrathymic) T cells, as tissues react thus: thymus, 0–5 percent; spleen, 25 percent; lymph nodes, 69 percent; bone marrow, 15 percent. These reactions agree with the proportion of (Thy⁺, Ig^–) cells present in these tissues. Cortisone-resistant thymus cells were positive. Absorption studies with thymus cells demonstrated the sparse or absent representation of Ly-6.2 on intrathymic T cells. Examination of spleen and lymph node cells from T cell-depleted C57BL/6 mice (after in vitro treatment with anti-Thy-1 serum or examination of tissues of C57BL/6-nu/nu mice) also showed a depletion of Ly-6.2⁺ cells. Conversely, removal of Ig⁺ B cells, which caused a relative increase in the number of T cells in the residual population, also increased the number of Ly-6.2⁺ cells. Additive effects of anti-Thy-1.2 and anti-Ly-6.2 could not be demonstrated, which suggests that the same population was Thy-1.2⁺, Ly-6.2⁺. However, additive effects could be shown with an anti-Ia serum and anti-Ly-6.2. The Ly-6.2 specificity is not found on red cells, liver, brain, or antibody-forming cells, but has been identified on a T-cell (but not B-cell) tumor and on kidney. Ly-6.2 can therefore be considered to be a marker for peripheral T cells, and it differs from the Thy-1 and the Ly-1,2,3, and 5 specificities in its relative absence from the thymus.     

Immunoglobulin of T lymphoma cells. Biosynthesis, surface representation, and partial characterization.

Four cloned continuously cultured mouse T lymphoma cell lines, WEHI-22.1, WEHI-7.1, S49.1, and EL-4.1, were examined for immunoglobulin biosynthesis and the presence of immunoglobulin on the cell surface. Incorporation of [-3H]leucine into cellular proteins followed by serological analysis showed that immunoglobulin constituted between 0.1 and 1.1 percent of protein synthesized by the different cell lines during a 6-hr period. Under the same conditions cultured cells of nonlymphoid origin, the mastocytoma P-815 X-2.1, did not synthesize any detectable immunoglobulin. Lactoperoxidase-catalyzed radioiodination was used to label proteins on the surface of viable lymphoma and mastocytoma cells. Although the lymphoma lines lacked immunoglobulin as assessed by fluorescent antibody staining, immunoglobulin was detected in surface proteins of all four lymphoma lines. Estimates of the number of immunoglobulin molecules on the cell surface were 1.1 times 10-4/cell for S49.1 and EL-4.1, 1.7 times 10-4 for WEHI-7.1, and 4.3 times 10-4 for WEHI-22.1. Electrophoretic mobilities in sodium dodecyl sulfate polyacrylamide gel indicated that intact cell surface immunoglobulin was slightly larger than IgG, and on disulfide bond reduction to dissociate into two components, one with the mobility of serum immunoglobulin light chain, the other with a mobility similar to that of mu heavy chain. The heavy chain from the T lymphoma cells possessed an apparent molecular weight of about 65,000 compared with 70,000 for mu chain, although both chains shared antigenic determinants characteristic of mu chains. These findings are interpreted as support for other reports that T lymphocytes carry immunoglobulin on their surface and as direct evidence that thymus-derived lymphoid cells synthesize an immunoglobulin resembling the 7S subunit of IgM.     

Activation of murine T lymphocytes with monoclonal antibodies: detection on Lyt-2+ cells of an antigen not associated with the T cell receptor complex but involved in T cell activation

A new T cell molecule defined by the mAb 143-4-2 has been identified that is involved in T cell activation. The expression of the 143-4-2-defined epitope is linked to the previously characterized Ly-6 locus and restricted to bone marrow cells and to a subset of peripheral Lyt-2+ cells. In comparison to other anti-Ly-6.2 mAb, the 143-4-2 mAb appears to be directed at an allogeneic determinant of the Ly-6.2C molecule. The anti-Ly-6.2C antibody can promote the lysis of antigen-non-bearing target cells by alloreactive CTL clones, and in the presence of cofactors (PMA or IL 2) induces a subset of Lyt-2+ cells to proliferate, perhaps through an autocrine pathway. Although the antibody described has antigen-like effects as described for anti-TcR complex reagents, studies performed with a recently derived anti-murine T3 mAb suggest that the Ly-6.2C molecule is not associated on the cell surface with components of the TcR complex. Nevertheless, cell surface expression of the TcR complex is required for optimal triggering of T cells via the Ly-6.2C molecule. Because Ly-6.2C determinants are expressed in bone marrow and not in the thymus, the possibility is considered that expression of this molecule identifies a distinct subset of extrathymically derived T cells.     

Regulation of B lymphocyte responses to IL-4 and IFN-gamma by activation through Ly-6A/E molecules

The Ly-6 family of cell surface molecules has previously been shown to participate in T cell activation. We show that Ly-6A/E proteins also modulated the response of normal B lymphocytes in three separate in vitro assays. First, unfractionated or small resting B cells proliferated when cultured with IFN-gamma, IL-4, and an anti-Ly-6A/E mAb. Second, this anti-Ly-6A/E mAb restored B cell proliferation responses that were inhibited when coculturing the B cells in IFN-gamma, IL-4, and anti-IgM. Third, anti-Ly-6A/E specifically up-regulated the cell surface expression of its own Ag, and this response was dependent upon co-stimulation with IFN-gamma. Mixing of T and B cells in culture suggested that T cells did not contribute substantially to the B cell proliferative response. Moreover, up-regulation of Ly-6A/E was observed for one B cell lymphoma, WEHI-231. Therefore, it appeared that modulation of B cell function by anti-Ly-6A/E was due to a direct effect of the mAb binding to the B cells. Taken together, these data suggest Ly-6A/E proteins are functional on B cells and may play a regulatory role in B cell activation.     

Effect of cytotoxic lymphocytes obtained by in vitro immunization with syngeneic lymphoma cells on pluripotent hemopoietic cells

The possibility of the presence of leukemia-associated antigens on pluripotent hemopoietic cells was studied with the aid of immune lymphocytes, cytotoxic against mouse syngeneic lymphoma cells. Cytotoxic lymphocytes were obtained during immunization in vitro of C57BL/6 mouse splenocytes by syngeneic T lymphoma EL-4 cells in the presence of interleukin-2. Specific cytotoxic activity of immune lymphocytes as regards EL-4 cells was not blocked by addition of normal bone marrow cells. Incubation of the bone marrow with immune killers did not lead to a decrease in the number of colony-forming units in the spleen. It was shown that using cytotoxic lymphocytes the total killing of lymphoma cells might be achieved in a mixture of bone marrow and lymphoma cells, whereas pluripotent precursor cells might be retained.     

The nature of the effector cells mediating mitogen-induced cellular cytotoxicity (MICC) and antibody-dependent cellular cytotoxicity (ADCC).

The nature of the cell types capable of mediating mitogen-induced cellular cytotoxicity (MICC) and antibody-dependent cellular cytotoxicity (ADCC) was investigated utilizing effector cells from athymic nude and euthymic heterozygous control littermate mice as well as Sephadex anti-Fab immunoabsorbent column purified spleen cell populations from normal (CS7BL/6) mice. Chicken erythrocytes (CRBC) and the mouse lymphoma, EL-4, were used as target cells in both cytotoxicity assays. MICC utilizing CRBC targets was mediated by several effector cell types whereas MICC utilizing EL-4 lymphoma targets was T-cell dependent. ADCC against both CRBC and EL-4 lymphoma targets occurred independently of the presence of T-cells. In addition, effector cell populations incapable of mediating MICC against EL-4 lymphoma targets were capable of mediating ADCC against the same EL-4 targets. Thus, utilizing the appropriate target cells, EL-4 but not CRBC, a sharp distinction can be made between the effectors for ADCC and MICC: ADCC is T-cell independent while MICC is dependent on the presence of mature thymus-derived cells. Furthermore these studies demonstrate that the nature of the target cell employed in MICC and ADCC reactions plays a critical role in defining the types of effector cells capable of mediating these cytotoxicity reactions.     

Identification of a novel T cell surface disulfide-bonded dimer distinct from the alpha/beta antigen receptor

Hybridomas were prepared from the spleen of a BALB/c mouse immunized with EL-4 T lymphoma cells. One, designated A1, was found to secrete a monoclonal antibody that reacted with two T lymphoma cells of C57BL origin, EL-4 and C6VLB, but not with normal C57BL/6 splenocytes or thymocytes, C57BL/6 T cell clones, or other T or B lymphomas by complement-mediated cytotoxicity or indirect immunofluorescent staining. Monoclonal antibody (MAb) A1 precipitated a protein that migrated at 85 kD under nonreducing and 43 kD under reducing conditions. The fact that the antigen defined by MAb A1 was a disulfide-linked dimer, together with the essentially clone-specific distribution of the reactive epitope, raised the possibility that the antibody defined an epitope of the antigen receptor. However, several additional observations revealed that the antibody defined a distinct and novel T cell surface structure. MAb 124-40, previously shown to react with the antigen receptor of C6VLB cells, reacted with variants of C6VLB that failed to express the A1 epitope. Sequential immunoprecipitation indicated that MAb A1 and MAb 124-40 reacted with distinct molecular species on C6VLB cells. Endoglycosidase digestion showed that the structure reactive with MAb A1 was not derived from that reactive with MAb 124-40 by addition of N-linked oligosaccharide residues. Two-dimensional gel electrophoretic analysis of precipitates obtained from radioiodinated C6VLB cells with MAb 124-40 resolved the alpha and beta subunits of the antigen receptor. Similar analysis of precipitates obtained with MAb A1 revealed only a single basic chain under reducing conditions, although anomalous mobility suggestive of a second, more acidic chain was observed under nonreducing conditions. Two-dimensional maps of tyrosine-containing chymotryptic peptides of the proteins isolated with MAb A1 and MAb 124-40 were completely different, suggesting that the molecules shared no peptides and were distinct in primary structure. Finally, cross-linking studies performed with a cleavable reagent indicated that the A1 molecule, unlike the antigen receptor defined with MAb 124-40, was not associated with additional, T3-like structures on the surface of C6VLB cells. Although the MAb A1 was unreactive with normal cells in cytotoxicity or staining assays, a molecule of the appropriate size was immunoprecipitated in small amounts from lysates of radioiodinated normal spleen and thymus cells. These data indicate that MAb A1 defines a novel disulfide-linked T cell surface molecule distinct from the antigen receptor.     

Analysis of Ly-6.2-bearing murine lymphocyte subpopulations in relation to the T-lymphocyte markers,Thy-1, Lyt-1, and Lyt-2

Using immunofluorescence with a monoclonal anti-Ly-6.2 antibody and FACS analysis we have confirmed that the Ly-6.2 antigen is present on approximately 70% of mature T cells and B cells but on few immature lymphocytes. There is a wide range of antigen density among the Ly-6.2⁺ populations, with the mean density higher on T cells than B cells. Following Con A activation of splenocytes there was a sixfold increase in Ly-6.2 antigen density though approximately 20% of the activated lymphocytes were Ly-6.2^?. The increase in Ly-6.2 density was specific since similar density increases did not occur for the closely linked antigens ThB and H $\text{9}\text{25}$. By panning a predominantly T-cell population for Lyt-2-bearing cells, it was found that Lyt-2⁺ lymphocytes were either negative or dully staining for Ly-6.2. However, activated cells bearing the Lyt-2 antigen were all Ly-6.2 positive. Double-staining experiments showed that T cells which had high Ly-6.2 antigen densities also had high Thy-1 antigen densities. Corticosteroid-resistant thymocytes were highly enriched for Ly-6.2-bearing cells compared to untreated thymocytes and had staining profiles for Ly-6.2 which were similar to peripheral T cells, supporting the idea that steroid treatment selects for a phenotypically mature thymic population.     

Molecular cloning of cDNAs from androgen-independent mRNA species of DBA/2 mouse sub-maxillary glands

cDNA libraries have been constructed from mRNAs isolated from mature male DBA/2 mouse submaxillary glands. Several recombinant plasmids have been assigned to particular mRNA species and their in vitro translation products by HART and hybrid selection. Clones containing copies of two abundant mRNA species that showed no sexual dimorphism were selected for detailed characterisation. Nucleotide sequences determined from one series of clones define an 850 nucleotide mRNA encoding a polypeptide of 16.5 kd having an N-terminal signal sequence, an acidic core and four glycosylation sites. A second family of clones correspond to an mRNA of 800 nucleotides, the sequence of which can be interpreted as coding for an intracellular protein of 14.7 kd. Computer searches of protein and nucleic acid sequences have not revealed the identity of either of these submaxillary gland products.     

Multiple cytokine interactions regulate Ly-6E antigen expression: cooperative Ly-6E induction by IFNs, TNF, and IL-1 in a T cell lymphoma and in its induction-deficient variants.

The cell surface Ly-6E antigen, known to play a role in T cell activation, is up-regulated by IFNs. In the present study, we investigated the possible interactions between IFNs and other cytokines in this regulation. As a model system, we used the YAC T cell lymphoma, in which Ly-6E is normally absent but can be highly induced both at the mRNA and surface protein levels by IFN-gamma or IFN-alpha/beta. The combination of the two IFNs was found to result in markedly synergistic Ly-6E induction in this cell line. Moreover, mutants of YAC cells were isolated that did not respond to the Ly-6E-inducing action of IFN-gamma or IFN-alpha/beta alone but did respond to their combination. Such a synergistic interaction is consistent with the notion that the two IFN types utilize different intracellular mechanisms to induce Ly-6E expression. Ly-6E induction mediated by IFN-gamma or IFN-alpha/beta was also enhanced by cotreatment with TNF-alpha or IL-1 alpha, which by themselves had no detectable Ly-6E-inducing effect. These two cytokines similarly synergized with IFNs to trigger a response in several Ly-6E-induction-deficient mutants. However, their action could be dissociated in one mutant (B54) where the response to IFN-alpha/beta was enhanced by TNF-alpha, but not by IL-1 alpha. Altogether, these data indicate that Ly-6E antigen expression is regulated by the interaction of several inflammatory cytokines, which may provide a mechanism for the local modulation of T cell activation. The YAC cell mutants described here should facilitate further analysis of the molecular bases of Ly-6E regulation.     

Three different signals are required for the induction of cytolytic T lymphocytes from resting precursors

The requirement for the signals in induction of cytolytic T lymphocytes (CTL) has been investigated. C57BL/6 X CBA/T6 F1 spleen cells stimulated with the lectin leukoagglutinin (L-A) failed to show CTL activity in a PHA-facilitated assay, although L-A-activated splenic T cells were able to respond to T cell growth factor (TCGF). Concanavalin A (Con A) on the other hand was able to induce cytolytic activity from CTL-P, as well as to render splenic T cells responsive to TCGF. Furthermore, L-A-activated splenic T cells could generate cytolytic activity upon subsequent culture in secondary mixed leukocyte culture supernatant (2 degrees MLC SN). In contrast, EL-4-derived SN (EL-4 SN) was unable to induce cytolytic activity from L-A-activated spleen cells. In addition, proliferation of L-A-activated spleen cells cultured in EL-4 SN was similar to those cultured in 2 degrees MLC SN. Nonactivated spleen cells were totally unresponsive to both SN in proliferation and generation of CTL. Analysis of T cell clones for the production of a factor necessary for induction of cytolytic activity revealed that both cytolytic and noncytolytic T cell clones were able to produce a factor(s) for the generation of cytolytic activity from L-A-activated T cells. On the other hand, SN from certain antigen-stimulated T cell clones produced factors capable of inducing cytocytic activity by L-A-activated T cells only in the presence of EL-4 SN. Neither EL-4 SN nor cloned T cell SN alone had such a capacity. The nature of the necessary lymphokines in the SN from the clone cells or from the EL-4 is unknown. In the case of the EL-4 SN, it is not known whether the presence of TCGF plays a role or whether that role is perhaps more differentiative than proliferative. This study provides evidence that the induction of CTL from CTL-P can be dissociated into activation, which is required to render T cells responsive to second signals, and differentiation, which is mediated by two different factors.     

Investigations into the nature of Igh-V region-restricted T cell interactions by using antibodies to antigens on methylcholanthrene-induced sarcomas. I. Analysis of an Igh-V-restricted suppressor-inducer factor

We have previously demonstrated the relationship between antigens on BALB/c methylcholanthrene (MC)-induced fibrosarcomas and T cell regulatory molecules by using a variety of antisera raised to these sarcomas in BALB/c and BALB/c X C57BL/6 (CB6F1) mice. One such pool of antiserum, a CB6F1 anti-CMS 4 (Pool XIV) serum, was used to investigate the nature of the T cell regulatory structures recognized by these antibodies. Pool XIV antiserum was capable of blocking the induction of feedback suppression by Ly-1 TsiF, an SRBC-specific suppressor T cell factor secreted by Ly-1+, 2- I-J+ T cells. Ly-1 TsiF induces suppression by interacting with an Ly-1+,2+ I-J+ T cell target. Successful interaction of Ly-1 TsiF with its target cell requires genetic homology between inducer and target cells at the variable region of the immunoglobulin heavy chain gene complex (Igh-V). The addition of Pool XIV antiserum to primary in vitro anti-SRBC cultures resulted in blocking the ability of Ly-1 TsiF from Igha (BALB/c) and Ighj (CBA/J) mice to induce suppression on syngeneic cells, whereas suppression induced by Ly-1 TsiF in Ighb (B6), Ighc (DBA/2), Ighd (A/J), and Ighe (AKR) mice are unaffected by addition of the Pool XIV antiserum. The ability of Pool XIV antiserum to block Ly-1 TsiF activity is linked to the Igh region, because Pool XIV antiserum can block Ly-1 TsiF from BALB/c (H-2d, Igha) and the Igh congenic B.C9 (H-2b, Igha) while not affecting Ly-1 TsiF activity on B6 (H-2b, Ighb) or its Igh congenic C.B20 (H-2d, Ighb). In CB6F1 animals, Pool XIV antiserum could block the ability of CB6F1 Ly-1 TsiF to suppress BALB/c spleen cells but not B6 spleen cells. Conversely, Pool XIV antiserum could block the ability of BALB/c Ly-1 TsiF to suppress CB6F1 spleen cells, whereas B6 Ly-1 TsiF showed normal suppressive activity in the presence of Pool XIV antiserum. In contrast, Pool XIV was capable of blocking the ability of Ly-1 TsiF from BALB/c into CB6F1 bone marrow chimeras (BMC) to suppress both BALB/c and B6 mice, whereas the activity of Ly-1 TsiF from B6 into CB6F1 BMC on BALB/c or B6 spleen cells was unaffected by the addition of Pool XIV antiserum. We then investigated the molecular nature of the molecule recognized by Pool XIV antiserum on the Ly-1 TsiF.(ABSTRACT TRUNCATED AT 400 WORDS)