Properties of antigen-specific suppressive T-cell factor in the regulation of antibody response of the mouse. I. In vivo activity and immunochemical characterization

Sharing of Ia antigens between species. I. Detection of Ia specificities shared by rats and mice

A mouse anti-rat xenogeneic antiserum, B10.D2 anti-BN, has been found to react with a subpopulation of lymphoid cells of certain mouse strains. The corresponding alloantiserum, B10.D2 anti-B10.BR, reacted in analogous fashion with lymphoid cells of BN rats. In the case of the cross-reaction on mouse cells, mapping studies indicated that at least part of the reactivity was with the product of gene(s) determined by the I-A subregion of the H-2 complex. Chemical isolation studies with radiolabeled cell surface preparations indicated that the antigens detected in both mouse and rat had mol wt characteristic of Ia antigens (35,000 and 28,000 dalton molecules). Testing of fractionated spleen cell populations revealed that the cross-reactive antigens were expressed predominatly on B cells, but that a subpopulation of T cells were also reactive. Wider strain and species distribution studies are in progress to determine the extent of such Ia cross-reactions between species and to further assess the practical and theoretical importance of such cross-reactions.

Suppression of IgE antibody production in SJL mice. III. Characterization of a suppressor substance extracted from normal SJL spleen cells

SJL mice were immunized with 1 microng dinitrophenylated keyhole limpet hemocyanin in 1 mg Al(OH)3. The mice were infected 21 days later with 750 third stage larvae of Nippostrongylus brasiliensis. On day 35, 14 days after infection, they were injected with 1 microng DNP-N, brasiliensis extract (Nb) in 1 mg Al(OH)3. In order to obtain high titer and persistent anti-DNP IgE antibody the mice were irradiated (540 R) 1 day after injection of DNP-Nb. Suppression of anti-DNP IgE antibody production was induced by spleen cells from normal SJL mice. Suppression of IgE antibody response is also obtained by an extract from normal SJL spleen cells. The suppressor substance from normal SJL spleen cell extract is a heat-labile protein, and is not absorbed by anti-mouse immunoglobulin. The mol wt of this substance is larger than 300,000 daltons as determined by gel filtration on Sephadex G-200, but after ultracentifugation, the supernate still has suppressive activity on IgE antibody production.

Suppression of immunoglobulin synthesis and secretion by peripheral blood lymphocytes from normal donors

Concanavalin A (Con A)-treated peripheral blood lymphocytes from healthy volunteer donors have been shown to suppress proliferative responses associated with thymus-derived lymphocytes (T-cells). The present investigations demonstrate that peripheral blood lymphocytes incubated with Con A for 48 hr can abrogate pokeweed mitogen-stimulated differentiation of bone-marrow-derived (B) lymphocytes to immunoglobulin-synthesizing and -secreting plasma cells. This effect was manifested when washed Con A-treated peripheral blood lymphocytes were added to pokeweed mitogen-stimulated cocultures containing fresh autologous or allogeneic mononuclear cells, and it did not appear to involve cytotoxicity. Parallel control cultures consisting of mononuclear leukocytes incubated for 48 hr in the absence of Con A also had immunoglobulin suppressor activity in mixing experiments. This effect, however, was most pronounced when preincubated cells were added to fresh autologous pokeweed mitogen-stimulated peripheral blood lymphocytes. Cell mixtures containing peripheral blood lymphocytes demonstrated a spectrum of immunoregulatory effects ranging from suppression to enhancement of pokeweed mitogen-stimulated immunoglobulin synthesis and secretion. Several functional subclasses of suppressor cells that reflect varying levels of specific activity have thus been demonstrated in human beings. Moreover, a degree of genetic identity appears to be required for the expression of "weak" immunoregulatory influences.

Suppressor cell induction in vitro. III. Antigen-specific suppression by supernatants of suppressor cells

Antigen-specific suppressor T cells induced in vitro release, after a further period of culture in vitro with antigen factors into the supernatant which have suppressor activity. These suppressor factors (SF) have the same antigen specificity as the suppressor cells (SC). SF only works on the early phase of thymus-dependent responses in cultures. SF inhibits thymusdependent IgM, but not thymus-independent IgM responses in vitro. SF is is destroyed by proteolytic enzymes, and inactivated at 80 degrees C for 30 min. The release of SF from SC is dependent on the presence of antigen and metabolically active cells.

Binding to idiotypic determinants of large proportions of thymus-derived lymphocytes in idiotypically suppressed mice

All A/J mice immunized with a conjugate of p-azophenylarsonate groups to keyhole limpet hemocyanin produce antibodies against azophenylarsonate, some of which share a crossreactive idiotype. The appearance of the idiotype can be suppressed, without reducing the response against azophenylarsonate, by injecting rabbit anti-idiotypic antibodies prior to immunization. We have now observed that mice suppressed in this way, or by adoptive transfer of leukocytes from other suppressed mice, and then immunized with the hemocyanin-azophenylarsonate conjugate, possess high proportions (up to 14%) of lymphocytes that form rosettes with A/J erythrocytes coated with Fab fragments possessing the idiotype. Idiotypic specificity was demonstrated by various experiments. Most of all of the rosette-forming lymphocytes appear to be thymus-derived lymphocytes (T cells). Treatment of T cells with trypsin eliminated the capacity to form rosettes, which was restored on standing overnight in medium. Thus, the receptors are synthesized by the cells and are not passively adsorbed. Treatment of mice with anti-idiotypic antiserum without antigenic stimulation did not elicit substantial numbers of rosette-forming cells. The requirement for antigen suggests that antigen-idiotype complexes may be a stimulatory agent. A prolonged rest period after immunization of suppressed mice was required for the induction of high percentages of rosette-forming cells. Rosette formation provides a convenient method for studying factors that induce the formation of idiotype-specific T cells.

Ia antigenic specificities are oligosaccharide in nature: hapten-inhibition studies

We have previously reported that the Ia specificities, coded for by the I region within the H-2 complex, appear to consist predominantly of carbohydrate. This conclusion was reached by examining low molecular weight Ia-bearing oligosacharides isolated from mouse serum. We now report hapten-inhibition studies which indicate that the binding of both allogeneic and xenogeneic anti-Ia antibodies to the Ia glycoproteins found predominantly on B lymphocytes can be specifically inhibited by certain free sugars. Both inhibition assays revealed that the specificity for the following Ia antigens resides predominantly in the following sugars: (a) Ia.1: N-acetyl-D-mannosamine or related sugars; (b) Ia.3: alpha-D-galactose and related sugars; (c) Ia.7: L-fucose; and (d) Ia.15: N-acetyl-D-glucosamine. It seems likely that these sugars are found at the terminal nonreducing ends of the carbohydrate portion of the Ia-bearing glycoproteins present in the lymphocyte membrane. In contrast, several public and private H-2 antigenic specificities did not appear to be sugar defined. These studies imply that at least some of the Ia genes from both the I-A and I-C subregions of the I region code for glycosyl transferases which modify oligosaccharide structure and impart specificity to the Ia antigens by alteration of their terminal sugar residues.

Partial chemical characterization of Ia antigens derived from murine thymocytes

Previous chemical studies attempting to demonstrate Ia antigens on mouse thymocytes have given contradictory results. We attempted to resolve the question of whether Ia antigens exist on thymocytes (defined as thymus cells that bear a T cell marker) by isolating strain C3H thymocytes free of other contaminating cells using the fluorescence-activated cell sorter, and then chemically testing the purified populations for Ia antigens. Immunoglobulin-negative thymus cells and thymus cells selected with a rabbit antiserum to mouse brain were the two populations of thymocytes labeled with [3H]leucine after sorting. Radiolabeled proteins were solubilized with the non-ionic detergent Nonidet P-40, reacted with anti-Ia antiserum, and analyzed by electrophoresis on discontinuous sodium dodecyl sulfate/polyacrylamide gels. Ia antigens were recovered from both cell populations. These antigens were synthesized by thymocytes and were found on molecules composed of two chains of molecular weight 33,000 and 25,000, respectively, similar to Ia antigens derived from spleen cells. Assuming that all thymocytes bear similar amounts of Ia antigens, we estimated that they have approximately 1/50 the amount of Ia antigens that spleen cells do.

Generation of T-helper cells in vitro. II. Analysis of supernates derived from T-helper cell cultures

Supernates derived from in vitro generated T-helper cells have been analyzed for their capacity to substitute for T-cell carrier reactivity. T-helper cell supernates stimulate both a carrier-specific and nonspecific anti-DNP-PFC response to DNP-carrier conjugates in cultures of hapten-primed spleen cells. The carrier-specific and nonspecific activity can be distinguished by dosage optimum, antigen requirements, binding specificity for carrier, and in the requirement for additional splenic adherent accessory cell involvement. The active factors produced in this system are heat labile and sensitive to trypsin and periodate. They are removed by absorption with alloantisera directed toward the strain from which the supernate was derived but not by a variety of anti-immunoglobulin sera.

Products of the major histocompatibility complex and their relationship to the immune response

The genes of the major histocompatibility complex were first known for the part they played in transplant rejection. Recently, however, it has become clear that the products of that region have an important part to play in the control of the immune response, through their effects both on cooperative and on aggressive interactions between cells. It is now possible to guess at the mechanisms which may underly the association of some major histocompatibility antigens with disease.

Properties of the antigen-specific suppressive T-cell factor in the regulation of antibody response of the mouse. IV. Special subregion assignment of the gene(s) that codes for the suppressive T-cell factor in the H-2 histocompatibility complex

The locus of the gene that codes for the antigen-specific suppressive T-cell factor was determined to be in a new subregion "I-J" which locates between I-B and I-C subregions in the H-2 histocompatibility complex. This was shown by two different lines of evidence: (a) The absorbing capacity for the suppressive T-cell factor of several alloantisera against restricted I subregions did not correlate with their specificity for previously known Ia molecules which are coded for by genes in I-A and I-C subregions, but was associated with the specificity for the products of genes putatively present between I-B and I-C subregions. By the occurrence of special recombinant strains, i.e. B10.A(5R), B10.A(3R), B10.S(9R), and B10.HTT, which differ with respect to the I-J subregion, we were able to produce alloantisera which distinguish I-J subregion gene products. The absorption studies using these special alloantisera directed to I-J subregion clearly indicated that the suppressive T-cell factor is a product of I-J subregion gene(s), and that the molecule is distinct from known Ia molecules expressed on splenic B cells. (b) Taking advantage of the fact that there is a strict histocompatibility requirement for the effective suppression between the donor and recipient strains of the suppressive T-cell factor, we were able to determine the required identities of the genes in the H-2 complex existing among those present between I-B and I-C. Again, utilizing the T-cell factors obtained from special recombinant strains, i.e. B10.A(4R) and B10.A(5R), we were able to locate the gene that codes for the suppressive T-cell factor reactive only with relevant haplotype strains between I-B and I-C subregions. These results are most reasonably explained by the presence of a new subregion I-J which is specialized in coding for the suppressive T-cell factor as a different molecule from previously known Ia molecules.

Expression of Ia determinants on immunocompetent cells

The expression of Ia (immune response region-associated) antigens on the surface of lymphocyte subpopulations with defined immunological function has been investigated by negative selection of subpopulations with anti-Ia sera and complement. Ia determinants were found on both unprimed (IgM) and on primed (IgG) antibody-forming precursor cells. No Ia antigens were detected on the surface of helper T cells. In contrast, suppressor T cells were sensitive to treatment with anti-Ia sera and complement demonstrating the presence of Ia determinants on this T cell subpopulation.

The Ly-3 antigens on mouse thymocytes: immune precipitation and molecular weight characterization

Specific anti-Ly sera were employed to precipitate Ly antigens from Nonidet P-40 extracts of mouse thymocytes labeled with 125I using lactoperoxidase and with NaB3H4 using galactose oxidase. Thymocytes from mice of the congenic strains C57BL/6J (Ly-2.2, Ly-3.2 positive), C57BL/6Ly-2a, Ly-3a (Ly-2.1, Ly-3.1 positive) and C57BL/6-Ly-2a (Ly-2.1, Ly-3.2-positive) were used as sources of labeled antigens and as immune adsorbants to permit evaluation of the specificity of each anti-Ly serum employed. Results of sodium dodecyl sulfate-polyacrylamide gel electrophoresis (SDS-PAGE) under reducing conditions are consistent with the Ly-3.1 antigen containing a glycoprotein subunit with an apparent mol wt of 35,000 daltons. Specific precipitates obtained using anti-Ly-2.1 serum yielded SDS-PAGE profiles identical to that obtained with anti-Ly-3.1 serum, suggesting that the Ly-2 and Ly-3 antigens have the same molecular weight distribution. The relationships of these results to the observed close genetic and topological linkage of Ly-2 and Ly-3 and to the genetic linkage of these loci with the IB-peptide marker, a mouse Bk-region polymorphism, are discussed.

Properties of the antigen-specific suppressive T-cell factor in the regulation of antibody response of the mouse. III. Dual gene control of the T-cell-mediated suppression of the antibody response

The antigen-specific suppressive T-cell factor of mice, which had previously been shown to be an I region gene product, could effectively suppress the in vitro secondary antibody response of spleen cells from syngeneic or H-2 compatible mouse strains but not that of H-2 incompatible strains. The identities among genes in the left side half (K, I-A, and I-B) of the H-2 complex between the donor and recipient strains were found to be both necessary and sufficient for the induction of suppression. This suggests that the acceptor site for the suppressive T-cell factor is also determined by the gene present in the left side half of the H-2 complex. The cell type which expresses the acceptor site was found to be a subset of T cell. In general, the suppressive T-cell factor obtained from F1 mice could suppress the responses of both parental strains, and the parental factors could suppress the response of F1 mice. The results indicate that both suppressor and acceptor molecules are codominantly expressed on F1 T cells. There were found two types of defects in the expression of suppressor and acceptor molecules among mouse strains: A/J mice could not produce the suppressive T-cell factor despite that they could accept the factor produced by other H-2 compatible mouse strains. In contrast, all the B10 congeneic lines could produce the T-cell factor, but could not accept the factor produced by syngeneic and H-2 compatible non-B10 congeneic lines. The F1 hybrid of A/J and B10. A could both produce and accept the T-cell factor, and thus the expressions of suppressor and acceptor molecules were found to be dominant traits. These results indicate that the antigen-specific T-cell-mediated suppression is regulated by at least two genes both present in the H-2 complex, and that the complementation of these two genes is required for the induction of suppression.

PHA-induced activation of suppressor cells in normal human peripheral blood lymphocytes

Normal human peripheral blood and tonsil lymphocytes can be stimulated to proliferate by phytohemagglutinin (PHA). When cells cultured with this mitogen for 3 days were transferred fo fresh autologous lymphocytes in fresh medium with PHA, the mitogen response of the fresh lymphocytes was suppressed. The suppression required the presence of viable cells, in that culture supernatants alone were not inhibitory and cell extracts showed only marginal inhibition. Approximately equivalent numbers of previously stimulated cells were required to produce optimal suppression of the PHA response of fresh cells. Cells irradiated after PHA stimulation were as effective as nonirradiated cells is causing suppression. PHA-stimulated cells also inhibited concanavalinA-induced proliferation and a mixed lymphocyte reaction. However, PHA-stimulated cells only partially inhibited the response to pokeweed mitogen. The suppressive effects were fully retained by a nylon-wool-enriched T-cell fraction but not by a B-cell-enriched fraction.

Studies of Ia antigens on murine peritoneal macrophages

Murine peritoneal cells, both induced and noninduced, were examined for Ia antigens by a variety of techniques. Complement-mediated cytotoxicity and indirect immunofluorescence, analyzed by both visual microscopy and the fluorescence-activated cell sorter, detected Ia antigens on the surface of an average of 8% to 15% of cells with the morphologic and functional characteristics of macrophages. Internal radioisotope labeling studies showed that these antigens were actually synthesized by the macrophages. The antigens were borne on molecules which consisted of two coponents with apparent molecular weights of roughly 33,000 and 25,000 daltons. At least some of these molecules existed as a two-chain structure of 58,000 daltons linked by disulfide bonds. Although macrophage Ia antigens appeared to be structurally similar to the Ia antigens found on spleen cells, the radioisotope labeling studies indicated that the quantity of labeled Ia-bearing molecules isolated from peritoneal macrophages was at most 1/15 that found for B lymphocytes. In addition, anti-Ia antisera failed to inhibit the binding of heat-aggregated immunoglobulin to the Fc receptor of macrophage populations, similar to the low levels of Ia antigens found in T-lymphocyte populations. These studies suggest that Ia antigens exist only a subpopulation of peritoneal macrophages. Alternatively, all cells in the population might bear small amounts of Ia antigens with only a fraction having sufficient numbers of molecules to be detected by the assay systems used.

Role of macrophages in the generation of T helper cells. IV. Nature of genetically related factor derived from macrophages incubated with soluble antigens

Macrophages incubated with soluble antigens for 1-4 days release factors capable of inducing T helper cells. The nature and some of the properties of this genetically related macrophage factor (GRF) were investigated. By use of immunoadsorbents, GRF was shown to contain I region-coded products (Ia antigens) linked to a small antigenic fragment. The genes coding for the Ia antigens detectable in GRF lie in the I region of the H-2 complex. GRF does not react with anti-immunoglobulin, anti-C3 or anti-human beta2-microglobulin antisera, and its activity cannot be removed by antigen immunoadsorbents. GRF is heat-labile, sensitive to proteolytic enzymes and has a molecular weight of about 55 000 Daltons. The relationship of GRF to other factors known to contain I region-coded products and reported to be effective in cell cooperation is discussed.

Shared idiotypic determinants on B and T lymphocytes reactive against the same antigenic determinants. V. Biochemical and serological characteristics of naturally occurring, soluble antigen-binding T-lymphocyte-derived molecules

Normal rat lymphocyte populations house a high percentage of lymphocytes with idiotypic, antigen-binding receptors for the major histocompatibility complex antigens of the rat. These receptors can be isolated from normal serum or lymphocyte supernatants. Idiotypic, antigen-binding molecules released from normal Lewis lymphocytes were thus isolated using anti-(Lewis anti-DA) immuno adsorbents. Analysis by SDS polyacrylamide electrophoresis using molecules labeled by external or internal means (125I or 3H) demonstrated that B lymphocytes produce molecules of 'conventional' 7S-8S IgM type. T-lymphocyte-derived molecules had a molecular weight of around 150,000 and consisted of two chains of similar size. Such single chains would succumb to proteolysis by normal serum factors to yield fragments in the size range of 30,000-40,000 daltons. All three groups of T-cell-derived molecules expressed both antigen-binding and idiotypic markers. No evidence was obtained that any light chains are linked to the T-receptor molecules. Serological analysis of the T-cell molecules failed to prove the existence of any determinants of constant immunoglobulin type, nor did these molecules express antigenic markers of major histocompatibility complex types.

Structural characteristics of the alloantigens determined by the major histocompatibility complex of the guinea pig

The GPLA B and Ia (I region-associated) antigens are the products of genes found in the guinea pig major histocompatibility complex. Because of their importance in immune response phenomena, a structural study of these antigens was undertaken. [3H]leucine and [3H]fucose were internally incorporated into guinea pig lymph node cells. The GPLA B and Ia antigens were solubilized by the nonionic detergent Nonidet P-40, purified by affinity chromatography using an adsorbent column of lentil lectin, isolated by immunoprecipitation, and examined by discontinuous polyacrylamide-sodium dodecyl sulfate gel electrophoresis. The GPLA antigens B.1, B.2, B.3, and B.4, were shown to be glycoproteins of mol wt 40,000 daltons and to be noncovalently associated with a 12,000 dalton protein. The molecules bearing B.2 and B.3 in a B.2/B.3 heterozygote are shown to be separable, suggesting the antigenic determinant is a primary gene product. In addition, a new GPLA determinant, S, which resembles the B antigen in that it is found on a molecule of approximately 40,000 daltons, was studied. In a B.2/B.3 S+ animal the molecule bearing antigen S was shown to be independent of those bearing B.2 and B.3, providing evidence that the genes determining B and S are at separate loci. The Ia-bearing molecules identified by anti-Ia.2,4 are glycoproteins of mol wt 58,000 daltons which are composed of two subunits of 33,000 and 25,000 daltons, respectively, linked by disulfide bonds. The Ia-bearing molecules are independent of GPLA-bearing molecules, indicating different loci determining these antigens. By all criteria, the guinea pig GPLA B antigens appear homologous to the murine H-2D and H-2K antigens, while the guinea pig Ia antigens appear homologous to the Ia antigens of the mouse.

Specific B-cell antigens associated with gluten-sensitive enteropathy and dermatitis herpetiformis

Gluten-sensitive enteropathy (G.S.E.) and dermatitis herpetiformis (D.H.) are associated with an increased frequency of the histocompatability antigen HLA-B8. These diseases were found to be even more strongly associated with a specific B-lymphocyte surface antigen recognised by maternal antisera. Two antisera (B-1 and W-1) reacted with B lymphocytes form G.S.E. and D.H. patients. Antiserum B-1 reacted with cells from thirteen of sixteen G.S.E. patients and from fifteen of nineteen D.H. patients, while antiserum W-1 reacted with cells from fifteen of sixteen G.S.E. patients and from fifteen of fifteen D.H. patients. None of the sera tested reacted with B lymphocytes from thirty-seven normal individuals, whether or not they were HL8-B8 positive. The identification of this specific antigen provides further insight into the pathogenesis of G.S.E. and D.H. and might form the basic of a diagnostic test for these diseases.

The role of receptors for T cell products in antibody formation

Immunocompetent cell interactions are achieved via direct contact between functionally different cell types or via interactions between soluble factors elaborated by regulatory T cells and specific receptors on responding cells for the T cell factors. In either case, there exist certain restrictions with respect to the effective interactions, which depend on the state of differentiation and genetic background of the responding cell type. Such restrictions are considered to be mainly determined by the development and nature of the receptor site on responding cell types for different T cell factors, which is now refered to the "acceptor" for the T cell factors. The presence of such acceptor sites on different populations of both T and B cells has been demonstrated in various experimental systems, and they are now considered to be the site by which responding cells receive appropriate signal for destination of their further differentiation. We have tried to review the nature and possible role of acceptor sites on both B and T cells for different T cell factors with respect to the induction and regulation of immune responses. A special emphasis was put on the genetic nature of the acceptor site. The observed genetic restrictions in the acceptance of T cell factors by responding cells suggest that such restrictions are needed for meaningful and unmistakable communications between funcionally different immunocometent cells. Furthermore, the presence or absence of acceptor sites for certain T cell factors is supposed to be a very important factor for determination of the immune responsiveness of animals against certain antigens, and thus in some cases the Ir gene effect may predominantly affect the expression of acceptor site. Possible implications of acceptor site in the regulation of antibody response and in the network of immunocompetent cell interactions are discussed.

The Ia antigens

The Ia antigens constitute a polymorphic series of cell surface determinants. At present, their definition is mainly a genetic one, and thus any cell surface antigen which can be demonstrated to be encoded by a gene in the Ir region of the H-2 complex may be classified as an Ia antigen. There are presently three subregions of the I region defined on the basis of available recombinant haplotypes, and designated at I-A, I-B, AND I-C. Mapping of individual Ia specificities indicates that numerous specificities are determined by genes in the I-A subregion, several in the I-C subregion, and few, if any, in the I-B subregion. This may be a reflection of the state of the art, however, rather than an accurate assessment of the extent of polymorphism. The Ia antigens appear to be expressed preferentially on the B-cell subpopulation of lymphoid cells. However, with the use of sensitive techniques they have also been demonstrated on some T cells, on macrophages, on sperm cells, and on epidermal cells. The Ia antigens have also been demonstrated on several T-cell factors which appear to be involved in the immune response. Whether or not all of the Ia antigens thus localized are identical or represent overlapping specificities within the same sera remains in many cases to be determined. There are presently three ways of defining Ia specificities serologically: (1) by direct immunization between strains differing only in the I region; (2) by detection of shared Ia determinants using polyspecific sera which contain H-2K region and H-2D region antibodies but which are nevertheless specific only for Ia antigens when tested on target cells of other strains; and (3) by selective absorption of H-2K region and H-2D region antibodies from an H-2 antiserum by cells bearing these antigens but lacking (or relatively lacking Ia antigens. All three of these methods produce anti-Ia reagents of reasonable titer for use in both serological and functional experimentation. The definition of the specificity as an Ia specificity in each case requires the availability of appropriate recombinant strains to map the specificity to the Iregion. In addition, there are several correlative criteria which have been developed in order to detect Ia activity in alloantisera in the absence of the availability of appropriate recombinants for mapping of the specificity. These include the tissue distribution of the Ia antigens (namely, their predominant expression on the B-cell subpopulation), their characteristics molecular size, their association on the B-cell surface with the Fc receptor, and their lack of association with other products of the major histocompatibility complex as distinguished either chemically or by cocapping studies. These correlative criteria make it possible to distinguish probable anti-Ia reactivity in a variety of serological reactions, but the results must still be interpreted with caution until appropriate recombinants have been obtained which can map the specificities to the I region...