Antigen-specific helper function of cell-free T cell products bearing TCR V beta 8 determinants

Although the T cell receptor (TCR) alpha beta heterodimer and its encoding genes have been characterized, a cell-free form of this receptor, which is needed for the study of functional or ligand-binding properties of the receptor, has not previously been isolated. When the cell-free supernatant products of activated cloned T helper (TH) cells were found to mediate helper activity with antigen specificity identical to that of intact T cells, experiments were carried out to determine whether this functional activity was mediated by a cell-free form of TCR-related material. A disulfide-linked dimer indistinguishable from the T cell surface alpha beta heterodimer was precipitated from cell-free supernatants of cloned TH cells with F23.1, a monoclonal antibody specific for a TCR V beta 8 determinant. Moreover, when cell-free TH products were bound to and eluted from immobilized F23.1, these affinity-purified materials had antigen-specific and major histocompatibility complex-restricted helper activity that synergized with recombinant lymphokines in the generation of B cell antibody responses. These findings suggest that the factor isolated from T cell supernatants is a cell-free form of the TCR alpha beta dimer.

Activation of Lyt-2+ (CD8+) and L3T4+ (CD4+) T cell subsets by anti-receptor antibody

The mAb F23.1, specific for V beta 8-related determinants on the TCR, was used to study the requirements for TCR cross-linking and for accessory cells (AC) in the induction of proliferation or IL-2 responsiveness in L3T4+ (CD4+) and Lyt-2+ (CD8+) T cells. T cells were exposed in vitro to soluble native F23.1 antibody, to heteroconjugates composed of the Fab fragments of F23.1 linked to Fab fragments of antibodies specific for Ia determinants on AC, or to F23.1 immobilized on an insoluble matrix. Soluble F23.1 antibody-induced proliferation in naive T cells only in the presence of both AC and exogenous IL-2, and these responses were confined to Lyt-2+ T cells. In contrast, heteroconjugates capable of crosslinking F23.1+ TCR to AC surface Ia determinants were capable of inducing proliferation in both L3T4+ and Lyt-2+ T cells in the absence of added lymphokine. Moreover, binding to and presumably multi-valent crosslinking of the TCR by immobilized F23.1 was sufficient to induce proliferation in both Lyt-2+ and L3T4+ T cells in the absence of AC or exogenous IL-2. Further, it was found that the conditions necessary for T cell growth factor secretion paralleled closely those required for induction of T cell proliferation in the absence of added lymphokine, suggesting that production of endogenous lymphokine might be the limiting process for triggering of T cell proliferation. Taken together, these findings suggest that under optimal conditions of TCR cross-linking, TCR occupancy and cross-linking is sufficient to deliver all of the signals necessary to initiate proliferation in naive populations of both L3T4+ and Lyt-2+ T cells. However, when conditions for TCR signaling are suboptimal, as may be the case for normal Ag-mediated stimulation, a role for second signals delivered by AC or exogenous lymphokines can become critical for T cell activation.

Activation of Lyt-2+ (CD8+) and L3T4+ (CD4+) T cell subsets by anti-receptor antibody

The mAb F23.1, specific for V beta 8-related determinants on the TCR, was used to study the requirements for TCR cross-linking and for accessory cells (AC) in the induction of proliferation or IL-2 responsiveness in L3T4+ (CD4+) and Lyt-2+ (CD8+) T cells. T cells were exposed in vitro to soluble native F23.1 antibody, to heteroconjugates composed of the Fab fragments of F23.1 linked to Fab fragments of antibodies specific for Ia determinants on AC, or to F23.1 immobilized on an insoluble matrix. Soluble F23.1 antibody-induced proliferation in naive T cells only in the presence of both AC and exogenous IL-2, and these responses were confined to Lyt-2+ T cells. In contrast, heteroconjugates capable of crosslinking F23.1+ TCR to AC surface Ia determinants were capable of inducing proliferation in both L3T4+ and Lyt-2+ T cells in the absence of added lymphokine. Moreover, binding to and presumably multi-valent crosslinking of the TCR by immobilized F23.1 was sufficient to induce proliferation in both Lyt-2+ and L3T4+ T cells in the absence of AC or exogenous IL-2. Further, it was found that the conditions necessary for T cell growth factor secretion paralleled closely those required for induction of T cell proliferation in the absence of added lymphokine, suggesting that production of endogenous lymphokine might be the limiting process for triggering of T cell proliferation. Taken together, these findings suggest that under optimal conditions of TCR cross-linking, TCR occupancy and cross-linking is sufficient to deliver all of the signals necessary to initiate proliferation in naive populations of both L3T4+ and Lyt-2+ T cells. However, when conditions for TCR signaling are suboptimal, as may be the case for normal Ag-mediated stimulation, a role for second signals delivered by AC or exogenous lymphokines can become critical for T cell activation.

Effect of Mlsa on antigen presentation to class II-restricted T cells

The nature of the gene products encoded or regulated by the minor lymphocyte-stimulating (Mls) loci remains enigmatic despite extensive experimental evaluation. This work tested the hypothesis that the Mlsa genotype, when compared to the Mlsb genotype, facilitates Ag presentation to class II-restricted T cells. Titrated numbers of H-2-identical, Mls-disparate APC were used to stimulate proliferation of autoreactive, alloreactive, or Ag-specific class II-restricted T cell clones or lines. Apparent preferential presentation by Mlsa vs Mlsb APC obtained from H-2-identical strains was seen infrequently, and when observed, analysis with the use of APC from recombinant inbred lines revealed that preferential presentation did not correlate with the Mls genotype of the APC. These studies show that the Mlsa genotype does not influence overall Ag presentation to class II-restricted T cells.

Effect of Mlsa on antigen presentation to class II-restricted T cells

The nature of the gene products encoded or regulated by the minor lymphocyte-stimulating (Mls) loci remains enigmatic despite extensive experimental evaluation. This work tested the hypothesis that the Mlsa genotype, when compared to the Mlsb genotype, facilitates Ag presentation to class II-restricted T cells. Titrated numbers of H-2-identical, Mls-disparate APC were used to stimulate proliferation of autoreactive, alloreactive, or Ag-specific class II-restricted T cell clones or lines. Apparent preferential presentation by Mlsa vs Mlsb APC obtained from H-2-identical strains was seen infrequently, and when observed, analysis with the use of APC from recombinant inbred lines revealed that preferential presentation did not correlate with the Mls genotype of the APC. These studies show that the Mlsa genotype does not influence overall Ag presentation to class II-restricted T cells.

Preferential expression of the T-cell receptor V beta 3 gene by Mlsc reactive T cells

The precursor frequency of T cells specific for any given foreign antigen is, in general, extremely low. Prominent exceptions to this rule are the T cells that are specific for foreign major histocompatibility complex (MHC) products or for products of the minor lymphocyte stimulatory (Mls) genes in the mouse which are present at high frequencies. Here, we report a striking overlap or cross-reactivity between the T cells specific for the protein antigen pigeon cytochrome c in association with Ek alpha Ek beta and the set of T cells specific for Mlsc products. In addition, we demonstrate that the basis for this overlap is the predominant expression of one T-cell receptor (TCR) V beta gene, V beta 3, by T cells that recognize Mlsc products. These results indicate the importance of specific TCR alpha beta dimers in the recognition of Mlsc products and that positive or negative selection of T cells specific for Mls self-determinants may selectively alter the repertoire of T cells available for MHC-restricted recognition of foreign antigens.

The expression of Mlsc determinants on Mlsa, Mlsb, and Mlsx prototypic strains

In the mouse system, specific determinants other than major histocompatibility complex (MHC) gene products are capable of inducing strong primary proliferative responses in naive T cells. These determinants are encoded by at least two gene loci designated as minor lymphocyte stimulatory (Mls) loci. In order to elucidate the biological role of the Mls system, an effort has been initiated to clarify the fundamental immunogenetic characteristics of the Mls system. In this report, we describe the unexpected finding that Mlsc determinants are expressed on splenocytes of strains including those which have been used as prototypic examples of three other Mls types: Mlsa (DBA/2, DBA/1), Mlsb (BALB/c), and Mlsx (PL/J). The expression of Mlsc by these strains was demonstrated both by the response patterns of unprimed T cells from MHC-identical inbred or F1 hybrid strains and by the responses of a panel of Mls-specific T-cell clones. The experimental results reported here also suggest that the expression of Mls determinants may be influenced by multiple other genes, including MHC-linked genes.

T helper cell cytoplasmic granules. Exocytosis in response to activation via the T cell receptor

A series of class II MHC-restricted keyhole limpet hemocyanin-specific Th cell clones were examined for cytoplasmic granules by histochemical techniques and fractionation of their homogenates. All showed granules containing lysosomal enzymes and high levels of trypsin-like activity revealed by a N-alpha-benzyloxycarbonyl-L-lysine thiobenzyl ester-esterase assay. Using the latter as a marker for granule contents, granule secretion was observed in response to MHC-restricted, Ag-dependent signals presented in vitro, and correlated well with T cell activation as measured by proliferation. However, human rIL-2, a strong inducer of T cell proliferation, did not stimulate N-alpha-benzyloxycarbonyl-L-lysine thiobenzyl ester-esterase secretion by itself, nor did it influence the secretion mediated by Ag. Other factors found to induce secretion included immobilized antibodies directed against determinants of the TCR complex, as well as Con A. These results suggest that granule enzyme secretion is another indicator of TCR-mediated activation and provides a possible mechanism for Th cell function via a rapid, local delivery of granule contents to adjacent cells.

T helper cell cytoplasmic granules. Exocytosis in response to activation via the T cell receptor

A series of class II MHC-restricted keyhole limpet hemocyanin-specific Th cell clones were examined for cytoplasmic granules by histochemical techniques and fractionation of their homogenates. All showed granules containing lysosomal enzymes and high levels of trypsin-like activity revealed by a N-alpha-benzyloxycarbonyl-L-lysine thiobenzyl ester-esterase assay. Using the latter as a marker for granule contents, granule secretion was observed in response to MHC-restricted, Ag-dependent signals presented in vitro, and correlated well with T cell activation as measured by proliferation. However, human rIL-2, a strong inducer of T cell proliferation, did not stimulate N-alpha-benzyloxycarbonyl-L-lysine thiobenzyl ester-esterase secretion by itself, nor did it influence the secretion mediated by Ag. Other factors found to induce secretion included immobilized antibodies directed against determinants of the TCR complex, as well as Con A. These results suggest that granule enzyme secretion is another indicator of TCR-mediated activation and provides a possible mechanism for Th cell function via a rapid, local delivery of granule contents to adjacent cells.

T cell recognition of Mlsc. I. Influence of MHC gene products in Mlsc-specific T cell recognition

Monospecific T cell clones have been proven to be powerful tools for the characterization of T cell recognition in many Ag-specific as well as allo-specific T cell responses. In this report, in order to elucidate the mechanism of T cell recognition of minor stimulating locus Ag (Mlsc) determinants, Mlsc-specific cloned T cells were employed together with primary T cell responses to clarify the role of MHC-gene products in Mlsc-specific T cell recognition. The results indicated that T cells recognize Mlsc determinants in conjunction with I-region MHC gene products. Moreover, certain MHC haplotypes (e.g., H-2a and H-2k) appear to function efficiently in the "presentation" of Mlsc, whereas other haplotypes (e.g., H-2b and H-2q) function poorly if at all in presenting Mlsc. Experiments with the use of stimulators derived from F1 hybrids between the low stimulatory H-2b, Mlsc strain, C3H.SW, and a panel of Mlsb, H-2-different or intra-H-2 recombinant strains strongly suggested that expression of E alpha E beta molecules on stimulators plays a critical role for Mlsc stimulation. The functional importance of the E alpha E beta product in Mlsc recognition was further demonstrated by the ability of anti-E alpha monoclonal antibody to inhibit the response of cloned Mlsc-specific T cells. Inhibition of the same Mlsc-specific response by anti-A beta k antibody suggests that the A beta product may also play a role in T cell responses to Mlsc.

T cell recognition of Mlsc. I. Influence of MHC gene products in Mlsc-specific T cell recognition

Monospecific T cell clones have been proven to be powerful tools for the characterization of T cell recognition in many Ag-specific as well as allo-specific T cell responses. In this report, in order to elucidate the mechanism of T cell recognition of minor stimulating locus Ag (Mlsc) determinants, Mlsc-specific cloned T cells were employed together with primary T cell responses to clarify the role of MHC-gene products in Mlsc-specific T cell recognition. The results indicated that T cells recognize Mlsc determinants in conjunction with I-region MHC gene products. Moreover, certain MHC haplotypes (e.g., H-2a and H-2k) appear to function efficiently in the "presentation" of Mlsc, whereas other haplotypes (e.g., H-2b and H-2q) function poorly if at all in presenting Mlsc. Experiments with the use of stimulators derived from F1 hybrids between the low stimulatory H-2b, Mlsc strain, C3H.SW, and a panel of Mlsb, H-2-different or intra-H-2 recombinant strains strongly suggested that expression of E alpha E beta molecules on stimulators plays a critical role for Mlsc stimulation. The functional importance of the E alpha E beta product in Mlsc recognition was further demonstrated by the ability of anti-E alpha monoclonal antibody to inhibit the response of cloned Mlsc-specific T cells. Inhibition of the same Mlsc-specific response by anti-A beta k antibody suggests that the A beta product may also play a role in T cell responses to Mlsc.

Genetic relationships of Mlsa among polymorphic loci on distal mouse chromosome 1

The segregation of Mlsa with respect to genes localized distally on mouse chromosome 1 was examined in two sets of recombinant inbred (RI) strains. In order to localize Mlsa, a linkage map based on analysis of both interspecific backcross mice and multiple sets of RI strains was utilized: (centromere) - Ren-1,2 - 10 centimorgans (cM) - At-3 - 8cM-Apo-A2/Ly-17 - 2cM - Spna-1 - 4cM - Akp-1 - (telomere). The Mlsa allele was mapped to a region that extended just centromeric of Ly-17 (one crossover in 40 RI strains) to just telomeric of Spna-1 (no crossover in 40 RI strains). The identification of multiple polymorphic loci in the region of Mlsa should allow precise gene localization and assist in efforts to clone this gene.

A reappraisal of Mls genetics

Although the minor lymphocyte stimulatory locus (Mls) system was originally described as a single-locus, multiple-allelic system, allelism among the genes which encode Mlsa, Mlsc and Mlsd has not been demonstrated formally. In this report, genetic linkage between genes encoding Mlsa and Mlsc was studied by testing responses of unprimed T cells, as well as Mlsa- and Mlsc-specific T cell clones, to the progeny of (AKR/J x C3H/HeJ) F1 x B10.BR [(Mlsa x Mlsc)F1 x Mlsb] crosses. In addition, the configuration of Mls genes expressed by CBA/J (Mlsd) was evaluated by examining the response pattern of a panel of Mlsd-specific clones to (CBA/J x B10.BR)F1 x B10.BR stimulators. Results of these segregation analyses indicated that the Mls system is composed of at least two independent and unlinked genes which encode, respectively, the determinants identified as Mlsa and Mlsc.

Mls determinants recognized by T cells

The relationship among different minor lymphocyte stimulatory locus (Mls) determinants, Mlsa, Mlsb, Mlsc and Mlsd, remains unclear. Because of the high degree of cross-reactivity between Mlsa and Mlsd determinants, the weak stimulatory capacity of Mlsc, and the generally non-stimulatory nature of Mlsb, some investigators have recently suggested that Mls is composed of only a single expressed allele originally defined as the a and d alleles. In order to clarify the nature of Mls determinants, T cell clones positively selected for reactivity to the three stimulatory Mls determinants, Mlsa, Mlsc and Mlsd, were generated and their specificities defined by extensive genetic studies. The response pattern of these Mls-specific clones as well as the results of primary mixed lymphocyte responses indicated that: (i) Mlsa and Mlsc determinants recognized by T cells are distinct, demonstrating that polymorphism does exist within the Mls system; and (ii) Mlsd is not an independent Mls type since Mlsd determinants are composed of Mlsa and Mlsc determinants and anti-Mlsd T cell responses are, in fact, the sum of anti-Mlsa and anti-Mlsc responses.

Analysis of two distinct B cell activation pathways mediated by a monoclonal T helper cell. II. T helper cell secretion of interleukin 4 selectively inhibits antigen-specific B cell activation by cognate, but not noncognate, interactions with T cells

A single monoclonal T helper (Th) clone can activate B cells in two distinct pathways; a cognate pathway requiring a major histocompatibility complex (MHC)-restricted T-B cell interaction, and a noncognate pathway not requiring an MHC-restricted T-B cell interaction. The present study was undertaken to investigate whether Th cells mediating a given immune response provide further regulatory function to B cells other than helper function. It was demonstrated that conditions of high antigen concentration which activate a noncognate B cell activation pathway simultaneously inhibit IgG responses. The inhibition is shown to be mediated by the T cell factor interleukin 4, produced by activated cloned Th cells. The inhibitory effect of this factor is directed to B cells and is MHC-unrestricted, antigen-nonspecific, and IgG class-specific. In addition to being susceptible to the effects of augmenting cells and suppressor cells, cloned Th cell populations can therefore themselves function as regulatory cells to inhibit IgG responses when stimulated with high dose of specific antigen. These results indicate that Th cells function to regulate B cells both positively and negatively, depending upon the activation conditions.

Analysis of two distinct B cell activation pathways mediated by a monoclonal T helper cell. II. T helper cell secretion of interleukin 4 selectively inhibits antigen-specific B cell activation by cognate, but not noncognate, interactions with T cells

A single monoclonal T helper (Th) clone can activate B cells in two distinct pathways; a cognate pathway requiring a major histocompatibility complex (MHC)-restricted T-B cell interaction, and a noncognate pathway not requiring an MHC-restricted T-B cell interaction. The present study was undertaken to investigate whether Th cells mediating a given immune response provide further regulatory function to B cells other than helper function. It was demonstrated that conditions of high antigen concentration which activate a noncognate B cell activation pathway simultaneously inhibit IgG responses. The inhibition is shown to be mediated by the T cell factor interleukin 4, produced by activated cloned Th cells. The inhibitory effect of this factor is directed to B cells and is MHC-unrestricted, antigen-nonspecific, and IgG class-specific. In addition to being susceptible to the effects of augmenting cells and suppressor cells, cloned Th cell populations can therefore themselves function as regulatory cells to inhibit IgG responses when stimulated with high dose of specific antigen. These results indicate that Th cells function to regulate B cells both positively and negatively, depending upon the activation conditions.

Mls is not a single gene, allelic system. Different stimulatory Mls determinants are the products of at least two nonallelic, unlinked genes

Mls determinants share with MHC products the unique property of stimulating T cells at extraordinarily high precursor frequencies. The Mls system was originally described as a single locus on chromosome 1, with four alleles, Mlsa, Mlsb, Mlsc, and Mlsd, that encode polymorphic cell surface structures. However, the fundamental issues of polymorphism and allelism in the Mls system remain controversial. To clarify these questions, a formal segregation analysis of the genes encoding Mlsa and Mlsc determinants was carried out by testing the capacity of spleen cells from progeny of (Mlsa X Mlsc)F1 X Mlsb breedings to stimulate responses by unprimed T cells and by Mlsa- and Mlsc-specific cloned T cells. The results of this analysis indicated that the gene encoding Mlsa determinants is neither allelic to nor linked to the gene encoding Mlsc determinants. Together with previous findings, these results also suggest that another strongly stimulatory type, Mlsd, in fact results from the independent expression of unlinked Mlsa and Mlsc gene products. Based on these observations, it is concluded that, contrary to conventional concepts, the stimulatory phenotypes designated as Mlsa, Mlsc, and Mlsd can be accounted for by the independent expression of the products of at least two unlinked gene loci.

Cytotoxic T lymphocyte recognition of a xenogeneic major histocompatibility complex antigen expressed in transgenic mice

Introduction of a porcine major histocompatability complex (MHC) class I gene (PD1) into the genome of a C57BL/10 (B10) mouse has been shown to lead to cell surface expression of the porcine MHC antigen, SLAPD1 in a transgenic mouse. The PD1 product expressed on spleen cells from the transgenic mice stimulated B10 spleen cells in a mixed lymphocyte culture to generate PD1-specific cytotoxic T lymphocytes (CTL). The CTL were PD1 specific since they lysed transgenic splenic blast cells and PD1-transfected L cells, but not B10 blasts or control L cells. The CTL were L3T4-, Lyt-2+ and their activity was partially inhibited by either anti-Lyt-2 antibody or by anti-swine MHC alloantibodies. The repertoire of responding B10 anti-transgenic CTL was assessed by examining their cross-reactivity on a series of murine allogeneic targets. The B10 anti-transgenic CTL showed some cross-reactivity on conventional allogeneic targets, but reacted strongly on a series of mutant H-2Kbm blast cells. In addition, B10 anti-B6.cH-2bm6 CTL cross-reacted extensively on the transgenic target cells. These results demonstrated that normal B10 CTL possess a repertoire specific for the products of the xenogeneic class I gene PD1, that this repertoire is cross-reactive with the conventional alloreactive CTL repertoire, and that there exists an unanticipated relationship between PD1-specific CTL and CTL specific for Kb mutant determinants.

Clonal analysis of the Mls system. A reappraisal of polymorphism and allelism among Mlsa, Mlsc, and Mlsd

Only two sets of antigenic determinants are recognized by T lymphocytes at uniquely high precursor frequencies: those encoded by the MHC and those encoded by Mls. The structural as well as functional characteristics of MHC products have been extensively analyzed. In contrast, little information concerning the nature of Mls genes or their products is available. Although it was originally described (5, 6) that the Mls locus on chromosome 1 is composed of four alleles that encode polymorphic cell surface structures, the issues of polymorphism and allelism in the Mls system have been controversial for some time. In the present study, T cell clones were generated by continuous stimulation of B10.BR (H-2k, Mlsb) T cells by CBA/J (H-2k, Mlsd) stimulators and they were used to analyze the relationship of putative Mlsa, Mlsc, and Mlsd determinants. All clones proliferated in response to determinants expressed by CBA/J stimulators. In addition, each of these clones exhibited a second reactivity to either AKR/J (H-2k, Mlsa) or C3H/HeJ (H-2k, Mlsc) stimulators. No clone responded to both AKR/J and C3H/HeJ. These second specificities were defined to be for Mlsa or Mlsc determinants, respectively, by the response patterns of clones and unprimed T cells to stimulators derived from congenic strains, recombinant inbred (RI) strains, and backcross mice. Moreover, a segregation analysis of the (CBA/J X B10.BR)F1 X B10.BR backcross indicated that the Mlsa-like and Mlsc-like determinants expressed on CBA/J (Mlsd) cells are in fact encoded by nonallelic, unlinked genes. These findings suggest a new concept of the polymorphism and genetics of the Mls system. It is proposed that two distinct and nonallelic gene products express, respectively, the noncrossreacting Mlsa and Mlsc determinants, and that the Mlsd phenotype does not represent an independent genotype but rather reflects the concurrent expression of Mlsa and Mlsc. The Mls system, therefore, consists of at least two systems that are distinct both genetically and antigenically, and that may be of different biologic or physiologic significance as well.

Different specificities of cloned T cells assessed by in vitro proliferation assays and by the ability to mediate skin graft rejection in vivo

The experiments presented here have compared the specificities of T cell clones as determined by in vitro proliferative responses and their specificities as reflected by their ability to mediate skin graft rejection in vivo. Two proliferative T cell clones with distinct in vitro specificities were evaluated for their ability to mediate rejection of skin grafts from C57BL/10 Scn nu/nu mice. Clone 14.11 (L3T4+, Lyt2-) was specifically stimulated to proliferate in vitro by I-Ad determinants, while clone 3.3.10 (also L3T4+, Lyt2-) recognized M1sa products in the context of MHC-encoded cell surface determinants. The results demonstrate that both clone 14.11 and clone 3.3.10 T cells are capable of mediating rejection of DBA/2 (H-2d, M1sa) skin grafts from B10 nu/nu mice. Surprisingly, neither clone 3.3.10 T cells nor clone 14.11 T cells were found to be effective at rejecting skin grafts from the D1.C congenic donor strain of mice (also H-2d, M1sa) from B10 nu/nu mice. Further, clone 14.11 T cells were also found to be ineffective at rejecting B10.D2 (H-2d, M1sb) skin grafts from B10 nu/nu mice. These data indicate the existence of a striking dichotomy between the specificity of alloreactive T cell clones as determined by in vitro proliferation studies and their functional capabilities in vivo.

Analysis of two distinct B cell activation pathways mediated by a monoclonal T helper cell. I. MHC-restricted activation of B cells by an IL 2-dependent pathway

The present study was carried out to determine whether the MHC-restricted and MHC-unrestricted B cell activation pathways mediated by a single cloned Th cell are separable, and whether these two pathways are mediated by distinct mechanisms. It was demonstrated that the two B cell activating functions of a single cloned Th cell could be separated by their sensitivity to irradiation. It was shown that MHC-restricted B cell activation is mediated by a radiosensitive Th cell function, whereas MHC-unrestricted B cell activation is mediated by a radioresistant function of the same Th cell. In addition, it was shown that recombinant IL 2 can restore or replace the radiosensitive component of MHC-restricted cognate helper function.