Microenvironmental immunoregulation: possible role of contrasuppressor cells in maintaining immune responses in gut-associated lymphoid tissues

Vaccination strategies for mucosal immune responses

Mucosal administration of vaccines is an important approach to the induction of appropriate immune responses to microbial and other environmental antigens in systemic sites and peripheral blood as well as in most external mucosal surfaces. The development of specific antibody- or T-cell-mediated immunologic responses and the induction of mucosally induced systemic immunologic hyporesponsiveness (oral or mucosal tolerance) depend on complex sets of immunologic events, including the nature of the antigenic stimulation of specialized lymphoid structures in the host, antigen-induced activation of different populations of regulatory T cells (Th1 versus Th2), and the expression of proinflammatory and immunoregulatory cytokines. Availability of mucosal vaccines will provide a painless approach to deliver large numbers of vaccine antigens for human immunization. Currently, an average infant will receive 20 to 25 percutaneous injections for vaccination against different childhood infections by 18 months of age. It should be possible to develop for human use effective, nonliving, recombinant, replicating, transgenic, and microbial vector- or plant-based mucosal vaccines to prevent infections. Based on the experience with many dietary antigens, it is also possible to manipulate the mucosal immune system to induce systemic tolerance against environmental, dietary, and possibly other autoantigens associated with allergic and autoimmune disorders. Mucosal immunity offers new strategies to induce protective immune responses against a variety of infectious agents. Such immunization may also provide new prophylactic or therapeutic avenues in the control of autoimmune diseases in humans.

Vaccination strategies for mucosal immune responses

Mucosal administration of vaccines is an important approach to the induction of appropriate immune responses to microbial and other environmental antigens in systemic sites and peripheral blood as well as in most external mucosal surfaces. The development of specific antibody- or T-cell-mediated immunologic responses and the induction of mucosally induced systemic immunologic hyporesponsiveness (oral or mucosal tolerance) depend on complex sets of immunologic events, including the nature of the antigenic stimulation of specialized lymphoid structures in the host, antigen-induced activation of different populations of regulatory T cells (Th1 versus Th2), and the expression of proinflammatory and immunoregulatory cytokines. Availability of mucosal vaccines will provide a painless approach to deliver large numbers of vaccine antigens for human immunization. Currently, an average infant will receive 20 to 25 percutaneous injections for vaccination against different childhood infections by 18 months of age. It should be possible to develop for human use effective, nonliving, recombinant, replicating, transgenic, and microbial vector- or plant-based mucosal vaccines to prevent infections. Based on the experience with many dietary antigens, it is also possible to manipulate the mucosal immune system to induce systemic tolerance against environmental, dietary, and possibly other autoantigens associated with allergic and autoimmune disorders. Mucosal immunity offers new strategies to induce protective immune responses against a variety of infectious agents. Such immunization may also provide new prophylactic or therapeutic avenues in the control of autoimmune diseases in humans.

Experimental autoimmune uveitis: molecular mimicry and oral tolerance

Intraocular inflammatory disease or uveitis, which affects the uveal tract and the retina of the eyes in human, is the major cause of visual impairment. Experimental autoimmune uveitis (EAU) is a T-cell-mediated autoimmune disease directed against retinal proteins and has been studied in several mammalian species including subhuman primates as a model for human posterior uveitis. Autoimmune responses provoked by molecular mimicry occur when the nonself and host determinants are similar enough to cross-react yet different enough to break immunological tolerance, and is one of the proposed mechanisms for induction of autoimmune diseases. Therapeutic immunomodulatory strategies have been used to induce antigen-specific peripheral immune tolerance in animal models of T-cell-mediated autoimmune diseases by oral administration of autoantigens. Oral tolerance leads to unique mechanisms of tissue and disease-specific immunosuppression, which would circumvent the immunotherapeutic problem of multiple target tissue autoreactivity. Several groups have investigated the effects of delivering autoantigens across gastric mucosal surfaces. This review briefly discusses molecular mimicry and the mechanism of induction of oral tolerance with respect to immunopathogenesis of T-cell-mediated autoimmune disease in general and EAU in particular.

A review of the mechanisms of oral tolerance and immunotherapy

The induction of oral tolerance by oral immunization has been well recognized. Accumulated evidence shows that oral tolerance can be mediated by orally activated humoral and cellular factors. In animal models, the development of several T cell-mediated autoimmune diseases, such as multiple sclerosis, rheumatoid arthritis, uveitis and diabetes type 1 can be inhibited by oral immunization of the respective antigens. In allergy, oral administration of certain allergens can prevent and reduce both contact and atopic dermatitis. Oral tolerance to alloantigen also reduces graft rejection. In spite of these encouraging results, the usefulness of this approach for an alternative immunotherapy in humans needs to be investigated further.

The effect of delayed weaning on the development of oral tolerance to soya-bean protein in pigs

The antibody response to a dietary antigen (soya-bean protein) and the development of oral tolerance was studied in pigs in a family pen system where the piglets are left with their mothers and gradually wean themselves onto a soya-bean-based diet over a 12 week period. In the first experiment three groups of pigs (eight pigs/group) aged either 2, 8 or 13 weeks were immunized with soya-bean protein or ovalbumin (OvA; controls) intra-peritoneally (i.p.) in Quill A adjuvant and subsequently boosted 2 weeks later. All groups showed an IgG response to the injected antigens indicating lack of tolerance induction to the dietary antigen. Interestingly the groups injected with OvA showed an almost identical response to soya-bean protein as the groups injected with soya-bean protein. In a second experiment with a similar protocol, soya bean was withdrawn from the feed before immunization which resulted in lack of response to soya-bean protein in the groups injected with OvA and a lack of response to injected soya-bean protein in the 14-week-old group, indicating that systemic tolerance was established by 12 weeks of age. The results from the two experiments suggest a compartmentalized response to soya-bean protein i.e. local antibody production to dietary soya bean along with systemic tolerance to injected soya-bean protein. The work also suggests that delayed 'natural' weaning may avoid damaging hypersensitivity reactions associated with early weaning.

Food allergy: facts and fiction

Hypersensitivity reactions to dietary products are especially common in the pediatric population, yet food allergy as a distinct clinical entity remains poorly defined. The clinicopathologic features of this syndrome can vary considerably from patient to patient; no reliable diagnostic laboratory tests are available, and some of the treatment regimens are controversial. The pathogenesis of this condition is not well understood. An important factor is the role of the intestinal mucosal barrier in the regulation of uptake of dietary antigen from the intestinal tract. Recently, significant differences have been observed between the features of the immature newborn and the mature adult intestinal mucosal barriers. These findings may be of fundamental importance to the pathogenesis of food allergy and are currently an area of intense research.

Vicia villosa agglutinin separates freshly isolated Peyer's Patch T cells into interleukin 5- or interleukin 2-producing subsets

Murine L3T4 T cells freshly isolated from Peyer's Patch were fractionated based on differential adherence to Vicia villosa agglutinin (VVA). VVA adherent cells secreted IL-5, but not IL-2, after stimulation with Con A and IL-1. In striking contrast, VVA nonadherent PP L3T4 T cells secreted IL-2, but not IL-5, under the same conditions. In addition, supernatants from VVA adherent, but not from VVA nonadherent cells cultures, enhanced IgA secretion by LPS-stimulated splenic B cells to the same extent as purified IL-5. Thus, IL-5-producing T cells are present in PP in situ and may play an important role in the development of mucosal immunity. Further, differential adherence to VVA can be used to separate T cell populations that preferentially secrete IL-5 or IL-2.

Immunoglobulin secretion by isolated intestinal lymphocytes: spontaneous production and T-cell regulation in normal small intestine and in patients with coeliac disease

The in vitro secretion of immunoglobulins by small intestinal lymphocytes isolated from 47 patients with normal histology and 23 patients with treated and untreated coeliac disease was examined using an enzyme linked immunosorbent assay. In control patients, duodenal lymphocytes spontaneously secreted higher levels of IgM than jejunal lymphocytes (p less than 0.05). Significantly higher levels of both IgA (p less than 0.05) and IgM (p less than 0.001) were secreted by jejunal lymphocytes of 10 patients with untreated coeliac disease than cells isolated from normal jejunal tissue. IgM and IgA secretion by duodenal lymphocytes isolated from control patients was increased in a dose dependent manner by coculture with autologous peripheral blood T lymphocytes. This effect was not observed with jejunal lymphocytes of control or treated coeliac patients. Peripheral T-cells of untreated coeliac patients, however, showed significant helper effects (p less than 0.05) for IgM and IgA secretion by autologous jejunal lymphocytes. The results suggest that jejunal lymphocytes of patients with untreated coeliac disease show major differences in their capacity to synthesise and secrete immunoglobulins in vitro and the enhanced secretion might result from changes in T-cell immunoregulatory function.

Mucosal homeostasis: role of interleukins, isotype-specific factors and contrasuppression in the IgA response

Oral ingestion of antigen elicits immune responses at mucosal sites where humoral immunity is largely due to antibodies of the IgA isotype. This is often accompanied by suppression of systemic responses to the same antigen, a state termed oral tolerance. This IgA response is regulated by interactions between T cell subsets found at IgA inductive tissues, i.e., the gut-associated lymphoreticular tissue (GALT) or Peyer's patches (PP). PP T helper (Th) cells support IgA responses, and interleukins 5 (IL-5) and IL-6 can augment secretion of this isotype. Subsets of Th cells may also express Fc receptors for IgA (Fc alpha R) and secrete Fc alpha R as an IgA-binding factor (IBF alpha). Membrane-derived Fc alpha R is a glycoprotein of 38,000 M.W. and this molecule induces selective increases in IgA secreting cells (as determined by the ELISPOT assay) in PP B cell cultures. Fc alpha R+ T cell lines have been shown to secrete IBF alpha as well as IL-5 both of which promote IgA synthesis. Recombinant IL-5 (rIL-5) and rIL-6 induce IgA synthesis mainly by PP B cell blasts, and principally act on surface IgA-positive (sIgA+) B cells for these responses. Another form of mucosal regulation is provided by T contrasuppressor (Tcs) cells, which abrogate oral tolerance when adoptively transferred to mice and restore systemic responsiveness to the antigen sheep erythrocyte (SRBC). Tcs cells from mice systemically primed with SRBC support IgM and IgG subclass responses, while Tcs cells from orally primed mice support IgM, IgG subclass and IgA anti-SRBC responses. These Tcs cells are CD3+, CD4-, 8- and are antigen-specific. These regulatory cells may use the gamma-delta (gamma-delta) form of T cell receptor for antigen recognition.

Impaired non-specific suppressor-inducer T-cell activity in aged murine Peyer's patches, which can be corrected largely by IL-2 in vitro

To shed further light on the mechanism of age-associated T-cell-mediated immunoregulatory alterations in gut non-specific mucosal immune responses, we studied in vitro the function of a variety of concanavalin A (Con A)-activated immunoregulatory T-cell subsets derived from aged murine Peyer's patches (PP) (BALB/c > 24 months old) in the production of class-specific immunoglobulins (Ig) by young (5-8 months old) lipopolysaccharide (LPS)-activated PP B cells. The in vitro induction of Con A-activated PP helper (Th) [L3T4+ Lyt-2- Vicia villosa non-adherent (VV-)], suppressor (Ts) L3T4- Lyt-2+ (VV-), and contrasuppresor (Tcs) (L3T4+ Lyt-2- VV+) T cells were compared for the two age groups. The activities of aged PP-derived Ts and Tcs cells were greatly impaired, in contrast to minor defects in activity of the aged Th cells. The induction of effector Tcs cells, however, depended on the presence of functionally effective Ts cells. Recombinant IL-2 (rIL-2) could largely correct this impaired generation of aged Ts and Tcs cells. Next, we determined the activities of Ts inducer (Tsi) (L3T4+ Lyt-2- VV-) T cells in aged PP in vitro. The cell activity was considerably diminished in aged mice. Then, we tested in vitro whether rIL-2 could reconstitute the impaired generation of the aged non-specific Tsi cell. The function of the latter cell was largely restored by rIL-2. Thus, the major functional (intrinsic) defect present in immunocompetent T lymphocytes of murine aged PP was confined to the Tsi cell.

Preliminary characterization and distribution of vicia villosa binding cells in human tonsils

The lectin Vicia villosa (VV) has been used for the separation of human and murine contrasuppressor T cells. These cells were characterized in cryostat sections of human palatine tonsils by double staining with VV lectin and monoclonal antibodies to macrophages, lymphocytes and their subsets using a fluorescein-rhodamine technique. VV lectin had an affinity for the CD8+ subset of lymphocytes and for a subset of macrophages within the germinal centre. The number and distribution of VV lectin binding cells was studied in paraffin sections of formalin fixed tonsils by the avidin-biotin-peroxidase technique. Positive cells in the germinal centres, mantle, interfollicular zones and fibrous connective tissue septa were quantified using an image analyser. These were found in greatest density in the interfollicular zone, correlating with the known distribution of T cells in human palatine tonsils. The binding of VV lectin to a subset of macrophages appears not to have previously been described nor have VV lectin binding CD8+ lymphocytes been demonstrated in sections of human tissues.

Phenotype and functional properties of Vicia villosa agglutinin (VVA) binding T cells in patients with Crohn's disease: detection of contrasuppressor activity in patients lacking extra-intestinal manifestations, abscesses and fistulas

Patients with active Crohn's disease (CD) have significantly increased numbers of T cells binding the Vicia villosa agglutinin (VVA). In patients with CD these VVA+ T cells express either the CD4 or the CD8 determinants, while in normal controls the majority of VVA+ T cells are CD8+. VVA+ T cells are significantly decreased in number in the inflamed mucosa as compared to normal controls. However, in only a subgroup of the patients do the VVA+ T cells show contrasuppressor activity with respect to the IgA and total Ig secretion upon co-cultivation with autologous B cells. Regression analysis revealed that in all data presented, contrasuppression activity correlates significantly with the absence of extra-intestinal symptoms, abscesses and fistulas.

A novel mechanism for the selection of isotype-specific antibody responses: the role of intestinal T cells in the regulation of IgA synthesis by the anti-suppressor circuit

Within 6 hr of immunization the serum of mice contains a unique form of processed antigen, which consists of a complex of immunoglobulin (Ig) and antigen formed in the presence of a factor derived from the anti-suppressor inducer T cell. This complex binds to and activates the anti-suppressor effector T cell, which eventually leads to the inhibition of suppressor cell function. Both of these cells are present in the spleen (SPL) and play a role in the regulation of antibody responses. The purpose of these studies was to identify the anti-suppressor T cells in gut-associated lymphoid tissue and compare their function to their splenic counterparts. Inducer cells were detected in the Peyer's patches (PP), mesenteric lymph nodes but not in the intra-epithelial lymphocytes. The effector cells, which take up the complexes, were detected in PP and lamina propria lymphocytes but not in the intraepithelial or mesenteric lymph node lymphocytes. Furthermore, the uptake of the complexes correlated with the presence of T cells bearing Ia antigens. The PP and SPL anti-suppressor cells were compared for their ability to enhance the production of IgA and IgG. The data clearly showed that the product of the inducer cell, and the effector cell it activates, not only enhanced the antigen-specific responses but also selected for isotype-specific antibody responses. Cells from SPL enhanced IgG greater than IgA, whereas cells from PP selected for IgA. Thus, the presence in PP of cells in the anti-suppressor circuit and their ability to selectively promote IgA synthesis suggest that this regulatory mechanism plays a significant role in intestinal immune responses.

Down-regulation of suppressor cell induction

A unidirectional cascade of cell interactions has been described previously that involves at least three distinct populations of suppressor T cells (Ts) that interact in appropriate succession to mediate suppression of delayed hypersensitivity responses to the 4-hydroxy-3-nitrophenylacetyl hapten (NP). The present work focuses on the potential bidirectional effects of one suppressor factor and how it can regulate the homeostatic mechanisms that maintain this suppressor cell cascade. Specifically, the effects of prior administration of mice with a transducer suppressor factor (TsF2) on the generation of NP-specific TS1 suppressor cells were evaluated. It was observed that the TSF2 given 2-14 days prior to administration of the tolerogen (NP-coupled splenic-adherent cells) interfered with the development of inducer TS1 suppressor cells. This down-regulation of suppressor cell induction is mediated by a population of cells that have the following characteristics: NP-binding, Lyt-1+2-, L3T4+, I-J+, and Vicia villosa (VV)-adherent T cells (for convenience these cells are termed anti-suppressor cells). Furthermore, there are genetic restrictions (both H-2 and IgH) between TSF2 and host which control the generation of anti-suppressor cells. The results suggest that TSF2 may have a role in homeostatic mechanisms that regulate the NP-specific suppressor cell cascade. The relationship of anti-suppressor and contra-suppressor cells is discussed.

In vitro induction of a contrasuppressor immunoregulatory network by polyclonally activated T cells derived from murine Peyer's patches

Much evidence suggests that Peyer's patch (PP) lymphocytes are capable of mounting both humoral and cell-mediated immune responses to luminal antigenic stimuli. To shed further light on T-T and T-B cell interactions in gut mucosal immune-associated processes, we studied in vitro the effects of a variety of PP-derived concanavalin A (Con A)-activated immunoregulatory T-cell subsets on class-specific immunoglobulin (Ig) production by lipopolysaccharide (LPS)-activated PP-derived B cells. Particular attention was focused on induction of a contrasuppressor T-cell immunoregulatory network in the above in vitro system. Three types of immunoregulatory effector T cells, a helper T (Th) cell, a suppressor T (Ts) cell and a contrasuppressor T (Tcs) cell were developed and isolated. The results showed that B cell Ig production was under the regulation of these T cells, and the L3T4+ Lyt-2- T cell, which bound to Vicia villosa (VV), had a contrasuppressor effector function. In addition, a L3T4+ Lyt-2- VV- Ts inducer (Tsi) subset and a L3T4- Lyt-2+ VV- Ts effector subset also appeared to participate in the sequential development of the suppressor and, probably, contrasuppressor immunoregulatory networks, respectively. Thus, PP T cells are likely to execute their highly sophisticated immunoregulatory functions, not only in the helper and suppressor circuits but also in the contrasuppressor circuit in response to intraluminal non-specific stimuli. However, IgA isotype-specific Ig production appears to be controlled primarily by the isotype-specific helper circuit, not by the contrasuppressor circuit, in polyclonal LPS-stimulated gut mucosal immune responses.

Evidence for contrasuppression in patients with Crohn's disease

Patients with Crohn's disease (CD) have elevated numbers of Vicia villosa agglutinin (VVA) binding cells in the peripheral blood. These cells represent a major subset of activated peripheral T cells. VVA binding T lymphocytes express either the T8 or the T4 determinant on their cell surface. In contrast in normal controls only a minor subset of peripheral T cell expresses binding sites for VVA. The majority of these cells coexpress T8. VVA binding T cells display no helper activity. Only in a subfraction of patients with CD and not in normal controls these cells mediate contrasuppressor activity for Ig and in particular for IgA. This subgroup of patients is characterized by the lack of extramucosal manifestations. It has now been shown that VVA binding T cells in their majority do not possess phenotypic features of helper inducer cells. This further supports the hypothesis of their involvement in contrasuppression. Moreover it was shown that IgA produced in the presence of VVA binding T cells is IgA1 and IgA2 (ratio 2:1) which are both modulated by VVA binding T cells.

Immunoregulatory defects in murine aged Peyer's patches

Aging effect on immunoregulatory processes in Peyer's patches (PP) from BALB/c mice of 3-4 months and 24-28 months was studied in vitro. The magnitude of isotype-specific Ig production by aged or young PP B cells in co-culture with young or old fresh PP T cells strongly suggested that the suppressor activity of PP T cells was impaired in aged mice. Further studies on in vitro induction of concanavalin A-activated T helper cells, T suppressor-inducer cells and T suppressor cells from aged PP indicated that the generation of T suppressor cells was largely impaired, in contrast to a minor defect(s) in that of T helper cells. The findings obtained here at least suggest that in aged PP, a T suppressor-inducer cell subset appears to be more selectively impaired in the aging process than the other lymphocyte subpopulations, which possess minor intrinsic functional defects. Thus, these abnormal T and B cell responses in PP could be responsible for the senescence-associated gut mucosal immunologic dysfunction.

Separation and Characterization of Leukocytes from the Intestine

This chapter discusses the methods for the separation and characterization of leukocytes from the intestine. Intestinal immune mechanisms must be studied directly and incorporated into models based on the observations made using intestinal cells and not just systemic cells. Numerous reports in the literature compare and contrast mucosal and systemic responses; however, despite their tissue-specific variations, the different tissues do interact. The procedures used to isolate cells from gut-associated lymphoid tissue (GALT) are traumatic or include enzyme digestion. A technique should be selected that is efficient yet gentle enough to provide viable and functional cells. Moreover, the cells contaminating leukocyte preparations can affect the functional assays. This necessitates that positive controls are put through the procedure or even added to intestinal pieces before isolation. If these cells demonstrate a positive response following this handling, then one is more confident of negative results observed in the isolated intestinal cells.

Virus-induced immunosuppression

Infection with a variety of viruses results in the suppression of the host's immune system. Several mechanisms thought to be responsible for this effect are discussed: infection and alteration of lymphocytes and macrophages, production of soluble suppressor factors, and the induction of suppressor cells. The clinical significance of virus-induced immunosuppression is also discussed.

Isotype-specific immunoregulation. Evidence for a distinct subset of T contrasuppressor cells for IgA responses in murine Peyer's patches

The ability of murine Peyer's patch (PP) T contrasuppressor cells (Tcs) to reverse oral tolerance to the T cell-dependent (TD) antigen SRBC was studied both in vivo and in vitro. C3H/HeJ mice given SRBC orally for 4 wk are not rendered tolerant to this antigen and were used as a source of PP Tcs cells for adoptive transfer to identically treated, orally tolerized C3H/HeN mice. Transfer of 10(4) or 5 X 10(4) V. villosa-adherent PP T cells resulted in splenic IgM, IgG, and mainly IgA responses in C3H/HeN mice challenged systemically with SRBC. The T cell responsible was Lyt-1+, 2-, L3T4-, I-JK+ and V. villosa lectin-adherent, all characteristics of mature effector Tcs cells. This C3H/HeJ PP Tcs cell subset was also effective when added to in vitro cultures of tolerized spleen cells derived from SRBC-fed, C3H/HeN mice. Interestingly, C3H/HeJ PP Tcs cells restored mainly IgA responses when transferred in vivo or when added to suppressed C3H/HeN splenic cultures. Comparison of the functional activity of Tcs cells derived from spleen or PP of orally immunized C3H/HeJ mice revealed that splenic Tcs cells supported responses of all 3 isotypes; however, PP Tcs cells yielded three-fourfold higher IgA responses, when compared with IgM or IgG anti-SRBC responses. Adherence of C3H/HeJ PP Tcs to an Fc alpha R+ T cell line derived from IgA-specific Th cells resulted in a nonadherent cell fraction that potentiated only IgM and IgG responses, while bound Tcs cells preferentially supported IgA responses. These results suggest that murine PP contain IgA-specific Tcs cells that allow IgA response induction in the presence of Ts cells that mediate oral tolerance.

Antigen presentation by epithelial cells of the rat small intestine. I. Kinetics, antigen specificity and blocking by anti-Ia antisera

Columnar epithelial cells (EC), isolated from the proximal small intestine of the rat, bind ovalbumin (OVA) by a non-specific, cold-inhibitable mechanism and continue to express Ia antigens after 24 hr culture in vitro. Lymph node T cells from rats immunized with OVA proliferate following 18 hr coculture with EC and OVA. This accessory cell function of EC is antigen-specific and is blocked by anti-Ia monoclonal antisera.

Antigen presentation by epithelial cells of the rat small intestine. II. Selective induction of suppressor T cells

Columnar epithelial cells (EC) isolated from the rat small intestine can present protein antigen in vitro, inducing proliferation of primed T cells. This system was used to examine the immunoregulatory function of T cells activated by presentation of ovalbumin (OVA) by EC. Rat lymph node T cells, sensitized to OVA in vivo and exposed to EC and OVA in vitro, suppressed the in vitro proliferative response of primed T cells to OVA. The suppression mediated by these modulated T cells (Tmod) was antigen-specific, was restricted during the induction phase by Ia antigens on the EC accessory cells, and was not affected by irradiation of Tmod. The generation of suppressor activity in Tmod was accompanied by an increase in expression of suppressor phenotype. It is suggested that a possible in vivo correlate of this phenomenon may play a role in the induction of suppressor T cells mediating systemic tolerance to dietary antigens.

Abrogation of oral tolerance by contrasuppressor T cells suggests the presence of regulatory T-cell networks in the mucosal immune system

Continuous ingestion of a thymus-dependent (TD) antigen differentially affects two compartments of the immune system. A secretory IgA antibody response is induced in mucosal tissues, concurrent with a state of antigen-specific systemic unresponsiveness to parenteral challenge, termed oral tolerance. The precise mechanisms whereby gut antigenic exposure induces oral tolerance are unknown, although T-suppressor cells, anti-idiotypic networks and immune complex formation have all been proposed. Here we show that the systemic unresponsiveness of mice made orally tolerant to the TD antigen sheep red blood cells (SRBC) is reversed by the adoptive transfer of Lyt-1+,2-, Vicia villosa lectin-adherent and I-J+ T cells derived from mice which are genetically resistant to the induction of oral tolerance to SRBC. This T-cell subpopulation has the characteristics of contrasuppressor effector T cells (Tcs). Small numbers of these Tcs cells reverse SRBC-specific tolerance both in vivo and in vitro. This finding offers new insight into the mechanisms of oral tolerance induction and maintenance, and suggests that a network of T cells are involved in the regulation of host responses to ingested antigens.

Comparative study of antigen binding T cells separated by Vicia villosa or streptococcal antigen and the effect of HLA class II antigens

The antigen binding capacity and function of T cells which adhere to the lectin Vicia villosa (VV) or to streptococcal antigen (SA) have been studied. VV-adherent T8+ cells (T8+ VV+) bind 125I-SA whereas VV non-adherent T8+ cells (T8+ VV-) bind little SA. Similarly, SA-adherent T8+ cells (T8+ SA+) bind 125I-SA specifically, whereas SA-non-adherent T8+ cells (T8+ SA-) show little binding of 125I-SA. The SA binding T8+ VV+ or T8+ SA+ cells can present the antigen to T4+ helper cells which generate helper factors and these enhance anti-DNP-antibodies, when incubated with mouse spleen cells and DNP-SA. Parallel reconstitution studies with either T8+ VV- or T8+ SA- cells have revealed that both subpopulations of cells can suppress T4+ helper cell activity. Further reconstitution experiments between the T8+ VV+ (or T8+ SA+) antigen presenting cells and T8+ VV- (or T8+ SA-) suppressor cells suggest that the former can also prevent the latter cells from inhibiting T4 helper cells, so as to function as contrasuppressor cells. Cross-over, reconstitution studies between the VV and SA separated cells have confirmed that the T8+ VV+ and T8+ SA+ cells have similar functions, in SA binding, presenting and contrasuppressor activities. The dose-response curve of binding 125I-SA to T8+ cells is dependent on the HLA-DR type of cells and the binding of 125I-tetanus toxoid was similar to that of 125I-SA. Whereas DRw6- T cells bind predominantly 1,000 ng SA or tetanus toxoid, DRw6+ T cells bind 1 ng and to a lesser extent 1,000 ng of either antigen. HLA-DRw6+ subjects can be considered as high responders for their T cells bind optimally at low concentrations of antigens which induce helper and contrasuppressor functions. In contrast, HLA-DRw6-subjects can be considered as low responders, as their T cells bind optimally at high concentrations of antigens and only the latter induce helper and contrasuppressor functions.

The role of a human antigen specific T8+ cell subset in antigen presentation, helper function and contrasuppression

Regulation of the human immune response was studied by sequential separation of subsets of T cells, followed by assessment of their helper and suppressor functions in a series of reconstitution experiments. T8+ lymphocytes were separated by panning on streptococcal antigen (SA) coated plates into T8+ SA-adherent cells (T8+SA+) and T8+ SA-non-adherent cells (T8+SA-). The helper and suppressor functions of the T8+SA+ and T8+SA- cells, reconstituted with T4+ helper cells were then studied by a direct antibody forming cell assay. T4+ cells will not induce helper activity by 1000 ng SA alone but require the accessory function of monocytes (Mo). However, replacing Mo by T8+SA+ cells will elicit a similar helper activity by T4+ cells and SA as that induced by Mo. In addition to the antigen-specific presentation and induction of helper activity, the T8+SA+ subset displays the properties of antigen-specific contrasuppressor cells. Thus, reconstitution of T4+ cells and T8+SA- (suppressor cells) with T8+SA+ and 1000 ng SA induces helper and no suppressor activity. Substitution of Mo for the T8+SA+ cells converts the helper to a predominantly suppressor-cell function. T8+SA- cells elicit suppression with 1 ng SA in the absence of accessory cells and reconstitution with Mo, T8+SA+ or T4+ cells failed to affect the suppressor activity. Total reconstitution of the four principle subsets of T4+, T8+SA+, T8+SA- cells and Mo elicited similar antigen dose-dependent responses as those of the unseparated mononuclear cells. It seems that all four cell subsets are required for optimal immunoregulation. We suggest that the T8+SA+ can present antigen to T4+ helper cells and induce helper activity, but in addition these cells can prevent the suppressor subset of T8+ cells from inhibiting T4+ helper cells and function as contrasuppressor cells. The mechanism of these functions is not known but HLA class II antigens might play an essential role in antigen binding, presentation and contrasuppression of the T8+SA+ cells, as the latter are significantly enriched in Ia+ cells.

Induction and control of the gastrointestinal immune system

The stimulation of cells in gut associated lymphoid tissue (GALT) by intestinal antigens can result either in immunity or tolerance to that antigen. The factors that determine which effect predominates are not understood, but the answer seems to lie in a better understanding of the cellular interactions and regulatory mechanisms in GALT. Induction of an immune response in GALT involves the same macrophage-T cell, T cell-T cell, and T cell-B cell interactions that have been described in other lymphoid tissues. Regulatory T cells have now been shown to play an important role in controlling the immune response to intestinal antigens. The presence of helper T cells specific for the IgA isotype in Peyer's patches may partly explain the old observation that the intestinal route is preferential for this antibody class. The stimulation of suppressor T cells in GALT is responsible for many instances in which tolerance rather than immunisation has followed antigen feeding. Although there is experimental evidence supporting the idea that mucosal IgA immunity and systemic IgG tolerance can occur concomitantly after antigen feeding, recent data obtained after the feeding of a variety of protein antigens indicate that this is not the usual result. More commonly either immunity or tolerance occurs concomitantly in both mucosal and systemic systems after antigen feeding, suggesting that the suppressor cells mediating oral tolerance also suppress mucosal IgA responses.

Effect of surgical trauma (gastrectomy) on cell-mediated and humoral responses in mice

Gastrectomy in mice affects the cell-mediated (CMI) and humoral immunity in a diverse fashion, such as CMI (contact sensitivity reaction) is severely impaired and antibody response is enhanced. Both effects are transient and disappear several days after surgery. While suppression of contact sensitivity is mediated by non-specific Ly 1-2+, L-J+ suppressor T cells generated by surgical stress, the mechanisms of enhancement of antibody response is unknown. We assume that the split unresponsiveness induced by surgical trauma has a clear survival advantage. Increased antibody production is the major defence mechanism against bacterial infections, while decrease of CMI prevents autoimmune response against altered (damaged) self structures.

[The gastrointestinal tract as an immunologic organ: the gut-associated immune system]

The gastrointestinal mucosa separates the intraluminal gastrointestinal fluid, which contains a high number of antigens from different sources, and prevents free access of antigens to the body. Simultaneously, it allows some vital host-environment interactions. A number of unspecific factors are important in preventing antigen invasion. The specific mucosal immunity is related to secretory IgA. IgA is derived from mucosal plasma cells after antigen-induced proliferation of its precursors in Peyer's patches. These IgA-positive B-lymphoblasts migrate through the systemic circulation and then "home" to the mucosa. IgA is translocated as a dimer to the gut lumen after attachment to the secretory component (SC). Part of it is excreted into the bile via small bile ducts after portal and possibly systemic circulation and binding to SC. T cells and mast cells are also considered to show migration and homing phenomena. In addition to the gut, some other mucosa-associated lymphoid tissues, (e.g. bronchus, mammary, salivary and lacrimal glands as well as the female genital tract), can participate in homing. Little is known about the local regulatory mechanisms, which allow an immunoglobulin class specifity of immune responses. Induction of local immunity and specific systemic tolerance seems to be a characteristic immune response of the gut-associated lymphoid tissue (GALT). The knowledge of the local immune system allows a better understanding of many aspects of gastrointestinal pathology, especially in immuno-inflammatory and immunoproliferative diseases as well as in gastrointestinal immunodeficiency syndromes.

Gut- and bronchus-associated lymphoid tissue

Bronchus-associated and gut-associated lymphoid tissues (BALT and GALT) have both functional and morphologic similarities and are involved in seeding lung, gut, and other mucosal sites with predominantly IgA-containing B cells. Both types of lymphoid tissue are engaged in the regulation and the controlled amplification of immune responses, which vary from positive mucosal responses in both mucosae and peripheral tissues to local mucosal responses and systemic tolerance. Their further involvement in provision of cells destined to reside in the epithelial compartment of the body appears likely but requires further investigation. Their role in the provision of precursors of mucosal mast cells must also be explored further, but some participation in this event appears likely. The mucosa-associated lymphoid tissue (MALT) system appears to be integrated with the systemic immune system but may be considered as separate from it in several functional ways.

Lyt phenotype, lymphocyte migration and the selective tissue positioning of mouse T cell sets

Functionally defined mouse Lyt-1 and Lyt-2 cell sets were selected from peripheral lymph nodes by cytotoxic elimination with anti-Lyt monoclonal antibodies. The selected populations were labelled with 51Cr or [3H]-adenosine and traced in syngeneic recipients at 1, 24, 45 and 65 hr after intravenous injection. Recovered radioactivity in recipient organs was measured by gamma-counting. The exact tissue positioning of the labelled cells was determined by autoradiography and labelled cell counting in spleen, lymph node and Peyer's patch microenvironments. Selected cell sets differed from unselected T cells in two ways: (i) Selected Lyt-1 and Lyt-2 cells showed some decline in recovery from recipient lymph nodes at 24 hr after injection; (ii) Lyt-2 cells showed enhanced localization to Peyer's patches. Autoradiographic analysis of microenvironmental tissue positioning of [3H]-adenosine labelled cells confirmed a relatively higher localization of Lyt-2 cells in Peyer's patches than in lymph nodes. In both tissues, the majority of the labelled cells were found in T areas. In the spleen, a higher proportion of Lyt-2 cells was seen in T-independent follicular areas.

Contrasuppression: the second law of thymodynamics, revisited

The data discussed here touch upon several issues in the evolving story of T cell contrasuppression, the underlying theme being that of heterogeneity. First, there is the issue of function. We are considering here only those cells that affect the function of secretory differentiation. We have evidence that different contrasuppressor cells exist for clone growth, but have not yet studied them in the same depth as those for secretory differentiation. Second, there is the important issue of target cells. In this article by Green and Gershon it is pointed out that there is clear evidence that contrasuppressor effects can work by protecting helper cells from suppressor cell effects in vitro. On the other hand, direct additional inhibition of the suppressor cells themselves has not been excluded. The latter point is also true in our system. However, we must suppose for the sake of simplicity in many of our experiments that if suppressors are not the target of the contrasuppressor effects then the B cells themselves probably are. This is because the tumor cells engage in a spontaneous rate of growth and differentiation in the absence of help or suppression. When T cell-dependent, specifically triggered effects reduce this spontaneous behavior, then a suppressive effect must have been delivered directly to the B cells. This is a simplifying assumption which is attractive, but since the experiments are carried out in vivo and thus may be affected by some factors that we have not yet recognized, we are not confident on its "intuitive" appeal. A third issue revolves around triggering specificity. One of our contrasuppressors exhibits the phenomenon of carrier crossreactivity (CRCS) and is thus behaving in accord with expectations aroused by Green and Gershon in this review. The other cell is apparently quite carrier specific (SCS). The meaning of this is not at all clear, but its potential significance may somehow be related to a sort of "mirror image" relationship of the two cells. Thus, for example, in other experiments not discussed here, we have noted that the CRCS binds to 315 protein-coated plates, but as noted here counteracts a suppressive effect which is generated by cells which do not adhere to these plates. In contrast to SCS does not bind to 315 plates and yet, as noted here, appears to counteract a suppressor effect generated by cells which do adhere to 315 plates.(ABSTRACT TRUNCATED AT 400 WORDS)

The IgA response: inductive aspects, regulatory cells, and effector functions

In this review, we have emphasized our current studies on the inductive aspects of the IgA immune response and homeostatic mechanisms involved in the induction of oral tolerance. By use of unique inbred mouse strains in restricted microbial environments, we have provided evidence for a central role of LPS in systemic unresponsiveness to orally encountered antigens. We have continued studies on characterization of GALT lymphoreticular cell types, including accessory cells, regulatory T-cells, and precursor IgA B-cells. We have placed recent emphasis on characterization of antigen-specific Th-cell clones derived from murine PP, which preferentially support IgA isotype responses. Relevant areas for continued research have been emphasized in this review.