Resistance to tolerance induction and age-dependent cellular changes in SJL mice

The PXR is a drug target for chronic inflammatory liver disease

PXR activators are used to treat pruritus in chronic inflammatory liver diseases such as primary biliary cirrhosis (PBC). The aims of this study were to determine whether PXR activators could have an additional benefit of inhibiting inflammation in the liver, and determine whether cyclosporin A - which more effectively prevents PBC recurrence in transplanted patients than FK506 - is a PXR activator. In SJL/J mice (which have constitutively high levels of hepatic portal tract inflammatory cell recruitment), feeding a PXR activator inhibited inflammation, TNFalpha and Il-1alpha mRNA expression in SJL/J-PXR(+/+), but not SJL/J-PXR(-/-). Monocytic cells - a major source of inflammatory mediators such as TNFalpha - expressed the PXR and PXR activators inhibited endotoxin-induced NF-kappaB activation and TNFalpha expression. PXR activation also inhibited endotoxin-stimulated TNFalpha secretion from liver monocytes/macrophages isolated from PXR(+/+) mice, but not from cells isolated from PXR(-/-) mice. To confirm that PXR activation inhibits NF-kappaB in vivo, 3x-kappaB-luc fibrotic mice (which express a luciferase gene regulated by NF-kappaB) were imaged after treatment with the hepatotoxin CCl(4). PXR activator inhibited the induction of hepatic NF-kappaB activity without affecting CCl(4) toxicity/hepatic damage. Using a PXR reporter gene assay, cyclosporin A - but not FK506 - was shown to be a direct PXR activator, and also to induce expression of the classic PXR-regulated CYP3A4 gene in human hepatocytes and in a cell line null for the FXR, a nuclear receptor with similar properties to the PXR.

CONCLUSION

PXR activation is anti-inflammatory in the liver and the effects of cyclosporin A in PBC disease recurrence may be mediated in part via the PXR. Since PXR activation promotes hepatocyte growth and is also anti-fibrogenic, the PXR may be an excellent drug target for the treatment of chronic inflammatory liver disease.

Suppressor cells and their precursors in A/J mice, tolerant to heterologous gamma globulin

The role of suppressor cells and of their precursors was examined in A/J mice, immunized or tolerized-immunized with rabbit gamma globulin. Antibody response and tolerance were assessed by antigen elimination, followed by an indirect plaque-forming assay. Reconstitution experiments were performed to estimate loss of cooperative capacity in thymus and spleen cells. Infectious tolerance was examined by reconstitution with mixtures of spleen or thymus cells of normal and tolerant donors. Infectious tolerance could not be detected after neonatally induced tolerance. It could be detected when tolerance was induced 11-16 days after birth. Under these circumstances, loss of cooperative capacity and increased capacity for infectious tolerance occurred rapidly over the first 2 days and reached completion by the 10th-20th day after administration of tolerogen. Thymectomy, after tolerance induction, resulted in relative recovery of responsiveness of spleen cells and loss of capacity for infectious tolerance. Pretreatment with cyclophosphamide resulted in a less profound state of unresponsiveness and in the disappearance of the capacity for infectious tolerance. Simultaneous treatment with tolerogen and colchicine also resulted in a less profound state of tolerance. This effect of colchicine was more profound when a low dose of tolerogen was used or when animals were thymectomized before administration of tolerogen and colchicine.

Neonatal tolerance induction in the thymus to MHC-class II-associated antigens. IV. Significance of intrathymic chimerism of blood-born Ia+ cells in Mls tolerance

The significance of thymus cell chimerism in the induction and maintenance of tolerance was investigated. Mls-1b BALB/c mice were neonatally tolerized by the intravenous administration of either bone marrow (BM) cells or peritoneal cavity (PerC) cells from Mls-1b/a (BALB/c x AKR) F1 mice. Tolerance was long-lasting in the BM cell group, but transient in the PerC cell group, probably because PerC cells lack hemopoietic stem cells required for a continuous supply of tolerance-inducing cells. The degree of anti-Mls-1a responsiveness of these BALB/c thymus cells was correlated with the degree of intrathymic distribution of the inoculated F1 cells. The effect of BM cell inoculation, resulting in a year-long deletion of Mls-1a-reactive V beta 6-bearing T cells is in marked contrast to that of PerC cell inoculation which causes only a transient loss of V beta 6+ mature thymocytes (for about 1 week after birth). This functional profile of the tolerant state correlates well with the degree and persistence of the intrathymic presence of F1 type Ia+ cells. The long-lasting presence of donor-derived cells throughout the thymus tissue in the BM cell group is also in marked contrast to the early disappearance of Ia+ cells (within 2-3 weeks) from the cortex and then from the medulla in the PerC cell group, although these Ia+ cells were once spread throughout the thymus tissue 4 days after the tolerance-inducing cell inoculation. Taken together with a failure to induce consistent unresponsiveness to Mls-1a determinants in Mls-1b thymocytes regenerating in Mls-1a-thymic epithelial environments, all the above data indicate that intrathymic chimerism caused by hemopoietic stem cell-derived MHC-class II-bearing cells is a requisite for the induction and maintenance of unresponsiveness by means of clonal deletion in experimentally as well as naturally induced tolerance to Mls determinants.

Strain polymorphism in progression of aging: changes in CD4, CD8 bearing subpopulations

Polymorphism of age-related changes in CD4 (L3T4) and CD8 (Lyt-2) determinants of spleen and thymus cells was assessed by fluorescence-activated flow cytometry. Cells from mice ranging from 5 weeks to greater than 2 years of age were examined. There is little age-related change in the proportion of CD4+ CD8- splenocytes in A, C57BL/6, DBA/1, DBA/2, and SJL mice (slopes 0.04, 0.06, 0.08, 0.17 and 0.17, respectively, when age in weeks was plotted against % of positive cells). Changes in the composition of the thymus are much more profound: CD4+ CD8+ cells of SJL mice decrease from 70% to less than 10% as the animals age from 5 to 69 weeks (slope -1.03), and in DBA/2 mice from 5 to 110 weeks (slope -0.88). While this decrease in CD4+ CD8+ cells occurs, there is a compensatory increase in CD4+ CD8- and CD4- CD8+ cells; this is a shift in the relative proportion of subpopulations rather than an increase in absolute cell numbers of a particular subpopulation. In contrast to the age-related changes of SJL and DBA/2 mice, there is relatively little change in the proportion of CD4+ CD8+ thymus cells in mice of strains C57BL/6, DBA/1 and A (slopes -0.03, -0.14 and -0.15, respectively).

Age-related changes in contact photosensitivity differ among mouse strains

There are differences among mouse strains in the age-related changes in reactivity to the contact photosensitizer tetrachlorosalicylanilide (TCSA). We found a tendency to lower reactions in older mice, with some strains showing declines from an early age (BALB/cJ, MRL/MpJ +/+, MRL/MpJ lpr/lpr and SJL/J). Others had increasing reactions until about 30-50 weeks of age before declining (DBA/1J, C3H/HeJ, and A/J) and one strain (C57BL/6J) had increased reactivity with age. There are also differences in the role of cyclophosphamide-sensitive T-suppressor cells in these age-related changes. In some mouse strains, BALB/cJ, C57BL/6J, A/J, DBA/1J and C3H/HeJ, age-related changes in reactivity to TCSA are independent of changes in cyclophosphamide-sensitive suppressor cells. In other strains, MRL/MpJ +/+, MRL/MpJ lpr/lpr and SJL/J, the development of cyclophosphamide-sensitive suppressor cells is responsible for the initial, though not later, stages of the age-related decline in reactivity.

Isotype-specific resistance against tolerance induction in SJL mice

Age-related changes in antibody response of SJL mice were examined in terms of isotype expression after treatment with immunogen or with immunogen, preceded by the molecule in normally tolerogenic form. We report here that tolerance induction and resistance to down regulation are isotype specific. Tolerance can be induced in terms of all detectable isotypes at the age of 5 weeks. In older SJL mice, tolerance to the carrier is found in IgM antibody, whereas there is resistance against down regulation in terms of IgG2a and IgG2b isotypes, and sensitization in terms of IgG3, IgG1, and IgA antibody. Furthermore, the degree of down regulation is determinant dependent. This was observed when older SJL mice, pretreated with the carrier in a normally tolerogenic form, were immunized with haptenated carrier and tested for their response to hapten and carrier determinants. In this case, IgA antibody shows tolerance to the hapten and sensitization by carrier determinants.

Strain-dependence and cellular aspects of the acceleration of age-dependent shift in class-specific helper and suppressor activity in the thymus of MRL/Mp mice by the LPR gene

Previous studies of the immunoregulatory activity of thymocytes from SJL/J mice have shown loss of suppressor activity for the antibody response by 24 weeks of age with appearance of helper activity. At the same time, suppressor cells developed which inhibit the generation of cytotoxic T lymphocytes (CTL). We now show a similar pattern of helper and suppressor activity in MRL/Mp mice. Presence of the lpr/lpr genotype significantly accelerated the onset of these changes in thymocyte activity. A similar pattern of thymocyte activity was not detected in C57B1/6 mice. In aged MRL-lpr mice, evidence of increased suppressor cell activity for the CTL response could be demonstrated in spleen, and the suppressor was sensitive to treatment with anti-thy 1.2 + complement. The magnitude of the deficiency in the CTL response in MRL-lpr mice was greater than could be accounted for by suppressor cell activity alone. Measurement of the frequency of CTL precursors (CTLP), the yield of CTL per CTLP, and the ability to produce and to respond to interleukin 2 (IL-2) indicated that a drop in CTLP frequency, subnormal generation of IL-2, and probably an intrinsic defect in the responsiveness of MRL-lpr CTLP to IL-2 was contributing to the defective CTL response. We were not able to link suppressor T cells with reduced responsiveness to IL-2. Ageing involves different patterns of change in immunoregulatory T-cell subsets in different strains of mice, depending on their genetic constitution. The general implications of this conclusion for prediction of immune dysfunction with age in genetically distinct members of an outbred population are discussed.

Autostimulatory lymphoid cells in aging mice: induction of syngeneic host-versus-graft reaction by radioresistant adherent cells in normal recipients

When young (6-week-old) BALB/c and also young C57BL/6 mice were inoculated into footpads with spleen cells from normal, syngeneic, aged donors, their response to the inoculum resulted in the enlargement of popliteal lymph nodes (PLN). The degree of PLN enlargement increased as the age of donor mice increased up to age one year. Spleen and lymph node cell populations were highly effective in eliciting the PLN enlargement. Thymus cells and bone marrow cells were moderately effective, but erythrocytes were ineffective. Resident peritoneal exudate cells and spleen adherent cells were much more effective than a whole spleen cell population. The syngeneic response seems to be attributable to a host-versus-graft reaction, since the PLN response was not affected by 2000 rad irradiation of inoculum cells and since the nylon wool-passed spleen T cell population was ineffective as the stimulator. The response was significantly reduced 3 weeks after thymectomy of recipients. PLN enlargement was also observed in older (7-month-old) mice which received spleen cells of younger mice. In this case, however, the response is ascribable to a graft-versus-host reaction, since the inoculation of radiosensitive spleen T cells into footpads resulted in the PLN enlargement. On the other hand, such a stimulatory activity was not found in either lymph node cells or thymus cells. These results suggest that the antigenicity of adherent cells changes with age and that the change is discernible by spleen-locating and short-lived T cells of young mice.

Sensitization of T and B cells by a normally tolerogenic macromolecule: the induction of unresponsiveness and of sensitization is polymorphic

Changes induced in B and T cells by tolerogen were experimentally defined in animals which resist down-regulation. Female A/J, C57BL/6J, MLR/MpJ-lpr/lpr, MLR/MpJ-+/+ and NZB/B1NJ mice were injected at various ages with a tolerogenic form of rabbit gamma globulin (sRGG) or were left uninjected, and all were then immunized with dinitrophenylated RGG (DNP-RGG) on alum. The degree of tolerance was estimated by measuring anti-DNP and anti-RGG spleen plaque-forming cell (PFC) numbers. In some cases, the state of T or B cells deduced from these experiments was further examined by cell transfer experiments. Four types of responsiveness to the tolerogenic form of RGG (sRGG) were distinguished: 1) persistent tolerance inducibility of T and B cells to tolerance induction (A/J); 2) T cells retaining tolerance-inducibility after initiation of age-dependent sensitization of B cells by the tolerogenic form of RGG (C57BL/6J, NZB/B1NJ); 3) age-dependent resistance of T cells to tolerance-induction and age-dependent sensitization of B cells by tolerogen, with the sensitization only manifesting itself in reactivity with T cells from immunized donors (SJL/J); and 4) sensitization of T cells by a usually tolerogenic form of RGG (MLR/MpJ-lpr-lpr). Thus, the development of resistance against tolerance induction is highly polymorphic, not only with respect to the age of onset, but also with respect to the cellular site of its first manifestation and the effect. Possible mechanisms of B-cell sensitization in sRGG-treated C57BL/6 and NZB mice are discussed in relation to the age-dependent emergence of a B-cell subpopulation and of non-specific helper T-cell factors.

Dietary fat alters the fatty acid composition of lymphocyte membranes and the rate at which suppressor capacity is lost

SJL/J mice were fed from conception two nutritionally adequate semi-purified diets that differed only in polyunsaturated to saturated fatty acid content. The effect of diet fat on the fatty acid composition of membranes from spleen and thymus cells was determined. Diet fat was found to significantly alter the fatty acid composition of lymphocyte membrane phosphatidylcholine and phosphatidylethanolamine. Diet also altered the degree of resistance against tolerance-induction.

Aging and the immune system

Information is presented on changes in the aging immune system. An analysis of separate streams of cellular aging in the inbred mouse is presented. It is demonstrated that there is extensive polymorphism in the aging of different types of executive and regulatory cell lines.

Polymorphism of T- and B-cell sensitization by aggregate-freed heterologous gamma-globulin

Female A/J, C57BL/6J, SJL/J, MRL/MpJ-+/+, MRL/MpJ-lpr/lpr, NZB/BINJ and male BXSB/MpJ mice were injected at various ages with a tolerogenic form of rabbit gamma-globulin, or were left untreated and all were then immunized with dinitrophenylated rabbit gamma-globulin. We could distinguish 4 types of responsiveness to sRGG: (1) persistent T-cell tolerance (A/J, MRL/MpJ-+/+); (2) persistent T-cell tolerance and age-dependent resistance and sensitization of B-cells to tolerance induction (C57BL/6J, NZB/BINJ); (3) decreasing T- and B-cell tolerance (SJL/J, male BXSB/MpJ); and (4) T-cell sensitization in older animals (MRL/MpJ-lpr/lpr). Suppressor capacity and its regeneration was examined in terms of colchicine and cyclophosphamide treatment. Colchicine increased the immune response to a much greater extent in 10-week-old than in 6-week-old MRL/MpJ-lpr/lpr mice and had little effect on MRL/MpJ-+/+ mice. It had a relatively small effect on 15-week-old NZB/B1NJ and a much greater effect on 6-week-old animals. The reason for these differences are discussed.

Sensitized spleen B-cells in tolerance-resisting SJL mice

A subpopulation of B-cells becomes sensitized in the 15-week-old SJL mouse, exposed to aggregate-freed rabbit gamma-globulin, which is highly tolerogenic when given to younger animals. This state of activation can only be expressed in collaboration with sensitized T-cells. The sensitized B-cell does not play an important role in the response of the intact 15-week-old SJL mouse, since there are no sensitized T-cells in the sRGG-treated animals.

Accelerated development of a Th-2 type factor in animals with and without an imbalance between help and suppression

A helper factor can be detected in antigen-treated supernatants from spleen T and adherent cells of sensitized animals. This factor promotes an indirect hapten-specific plaque forming response of B cells, irrespective of the identity of the carrier, i.e. provides the Th-2 type of help. Factor production increases with age and occurs most rapidly in strains known to have an accelerated decrease of suppressor capacity. The reason for the inverse correlation between suppressor capacity and the Th-2 type of helper factor is discussed.

Mechanisms involved in age-dependent decline of immune responsiveness and apparent resistance against tolerance induction in C57BL/6 mice

The plaque-forming antibody response of C57BL/6 mice to rabbit gamma globulin (RGG) decreases as a function of ae. RGG in tolerogenic form induces tolerance of young mice but sensitizes older animals. If antigen is administered together with lipopolysaccharide, the age-dependent decline in immune responsiveness is not observed, nor are older animals sensitized by tolerogen. The age-dependent decline in immune responsiveness is due to a loss of T helper capacity; sensitization by tolerogen is attributable to a subpopulation of B cells which becomes sensitized by the tolerogen. Older animals, treated with tolerogen, show a degree of central tolerance of T cells and a relatively slight, age-dependent diminution in colchicine- or cyclophosphamide-sensitive precursors of suppressor cells.

Accelerated development of Th-2 type of helper effect in MRL/MpJ-lpr/lpr and BXSB/MpJ mice

Age-dependent changes, during adult life, in suppressor capacity and B cell responsiveness have been known for some time. In this paper, an age dependent change in helper function, is being reported. A Th-2 type of helper effect can be observed in tissue culture of spleen cells from animals sensitized with a macromolecule. An indirect plaque-forming response to the hapten occurs when the spleen cells are exposed to the sensitizing macromolecule and to the hapten conjugated to a carrier, structurally unrelated to the sensitizing macromolecule. There is considerable polymorphism in the age at which this effect is first demonstrable and at which it reaches mature levels. Mice of inbred stains, with various defects in the suppressor cell circuit (SJL/J, MRL/MpJ-lpr/lpr, BXSB/MpJ show an accelerated development of Th-2 helper capacity in the indirect plaque-forming response.

Effect of cyclophosphamide on development of reticulum cell sarcoma in SJL/J mice

Studies to determine the effect of cyclophosphamide (CY) on the development of reticulum cell sarcoma (RCS) in SJL/J mice indicated a dependence on the duration of the test period. Age also appeared a factor of importance. Thus, a comparison of tumor incidences at 52 weeks of age showed maximal inhibition when CY was administered at 40 weeks, minimal inhibition when the drug was given at 30 weeks, and intermediate inhibition when treatment was initiated at 10 and 20 weeks. Consistent with these findings, long-term treatment of 40-week-old SJL/J mice with low doses of CY resulted in an increase in the mean survival time and in a reduction in the incidence of RCS.

The use of phagocytic peritoneal exudate cells as targets for the estimation of cytotoxic T cell activity

A new method for measuring cytotoxic T lymphocyte (CTL) activity has been developed with peritoneal exudate cells (PECs) as target. A monolayer of target PECs was labelled with Indian ink and exposed to CTL. After incubation, detached or damaged PECs were rinsed out of the target monolayer, and the remaining phagocytes were fragmented and dissolved by the addition of 2 N NaOH. The number of undamaged target cells was estimated by colorimetric reading of the amount of Indian ink in the phagocytes surviving CTL attack. In studies with several strains of mice, only PECs from the same strain as the stimulator cell donors used for CTL induction were damaged by CTL effectors. When CTLs generated against TNP-modified syngeneic stimulators were used as effectors TNP syngeneic PECs, but neither unmodified syngeneic PECs nor allogeneic TNP-PECs, were damaged. These results demonstrate the antigen specificity of immunolysis of PECs. Macrophage target cells were stable and showed no tendency to spontaneous cell damage during 24 h incubation without effector cells.

The effect of ribavirin and rifamycin SV on age-dependent changes of the immune system during the adult life of SJL mice

Ribavirin and Rifamycin SV, given from birth to adult life, arrest the loss of suppressor capacity, normally occurring in older SJL mice. The two drugs differ in their effect on Th-2-type help for the indirect plaque-forming response, i.e. for help which is independent of the carrier-hapten bridge. This type of help normally increases with age. Ribavirin treatment inhibits development of Th-2-type of help; Rifamycin SV does not interfere with the development of this type of help. Neither of the two drugs had an effect on the ability of SJL mice, not treated with aggregate-freed RGG, to make a plaque-forming response to aggregated RGG.

Diversity of age-dependent changes of antibody formation and of tolerance decrease

Age-dependent changes in immune response and in tolerance occur at different rates in inbred mice of different genetic backgrounds. The change in tolerizability is connected with the change in the balance of the suppressor helper circuit; the change in B-cells, at least in C57BL/6 mice, appears to be also due to a regulatory change. Changes in immune responsiveness and tolerance inducibility appear to vary independently of one another.

Resistance of MRL/Mp-lpr/lpr mice to tolerance induction

Mice of strain MRL/Mp-lpr/lpr develop resistance against tolerance induction with aggregate-freed rabbit gamma-globulin as early as 6 weeks of age and show very complete resistance when they are 10 weeks old; congenic mice MRL-Mp+/+ of 6 and 10 weeks can be rendered tolerant. By reconstitution experiments, it was shown that T cells from MRL/Mp-lpr/lpr are responsible for the resistance to tolerance induction. A cell which is not colchicine-sensitive contributes to tolerance of MRL/Mp-+/+, but not to MRL/Mp-lpr/lpr mice. Resistance of T cells against tolerance depends on expression of the lpr gene.

Age-dependent separation of class-specific suppressor cells in thymus of SJL/J mice

The thymus of SJL/J mice of age 3-6 weeks has been previously shown to contain suppressor cells that inhibit the antibody response to lymph node cells to SRBC. The effect of these suppressor cells disappear as the animals age (24 weeks or more). We find that these aged animals acquire thymic suppressor cells which suppress the generation of cytotoxic T-cells both in vitro and in vivo. Although such suppressors are not present in the thymuses of young SJL/J mice, suppression can be induced by treatment with estrogen and progesterone. The differentiation of functionally different suppressor cell populations in thymus may be affected by both age and hormonal status.

Effect of 1-beta-D-ribofuranosyl-1,2,4-triazole-3-carboxamide (ribavirin) on tolerance induction in SJL mice

The effect of 1-beta-D-ribofuranosyl-1,2,4-triazole-3-carboxamide (ribavirin) on tolerance induction with rabbit gamma globulin (RGG) was investigated. The treatment with ribavirin, initiated on the 4th and 17th day of life, did not reduce the immune response significantly, but provided a much more profound level of tolerance than could be achieved in animals not treated with ribavirin. When treatment with ribavirin was initiated at the age of 50 or 65 days, the response of immunized animals was reduced and the response of tolerized-immunized animals was increased. The mechanism, underlying the drug action, was discussed in terms of an effect or ribavirin on the balance between help and suppression; the possibility that a virus may be involved in age-dependent changes and that ribavirin prevents it proliferation, was considered.

A strain survey of age-dependent changes in antigen elimination, antibody formation and tolerance

A strain survey has been undertaken to examine polymorphism of age-dependent changes in antigen elimination, immune responsiveness and tolerance inducibility. In young mice of tested strains, other than C57BL/6J, aggregate-freed mouse Ig was eliminated faster than was the xenogeneic Ig(RGG); C57BL/6J was the only exception. The rate of elimination increased with age in all strains, tested. Antibody formation to RGG decreased in 3 (BALB/cJ, 129/J, DBA/2J) out of the eleven strains tested at 5--7 and 31--40 weeks of age; there was a considerable strain variation in the extent of this decrease. Age-dependent resistance to tolerance induction was observed in a fraction of animals of H-2 haplotypes, H-2k and H-2d.

Accelerated age-dependent decline in the T suppressor capacity of SJL mice

Tolerance induction with rabbit gamma-globulin was employed as a probe for age-dependent changes in suppressor capacity of SJL lymphoid cells. The tolerant state was assessed by loss of cooperative capacity and by infectious tolerance. The supply of precursor cells was assessed by thymectomy and by treatment with colchicine and cyclophosphamide, which have been reported to eliminate suppressor cells. Thymectomy, 16-18 days before tolerance induction, did not affect antibody response or tolerance inducibility; thymectomy, 33 days before tolerance induction, reduced both antibody response and tolerance inducibility. Colchicine, injected together with aggregate-freed rabbit gamma-globulin, inhibited tolerance induction partially in 35-day-old mice and completely in 106-day-old mice. Colchicine, given to younger mice, thymectomized 17 days before tolerance induction, prevented tolerance induction completely. A low dose of cyclophosphamide interfered with tolerance induction in older, but not in younger mice. A high dose of cyclophosphamide interfered with tolerance induction in thymus cells of younger and older mice. After thymectomy, there was a much more profound interference of a low dose of cyclophosphamide with tolerance induction. Results were discussed in terms of an age-dependent decline of thymus progenitors and of peripheral progenitors of suppressor cells.

Age-dependent changes in tolerizability with rabbit gamma-globulin in the Biozzi high and low-responder lines of mice

The antibody response to rabbit gamma-globulin (RGG) of high-responder, but not of low-responder Biozzi mice decreased with age. Injection with aggregate-freed RGG reduced the response of high-responder but not of low-responder mice to subsequent injections with aggregated RGG. This reduction in the antibody response, formed by high-responder mice, decreased with increasing age; aggregate-freed RGG appeared to sensitize 6-month-old low-responder mice to a subsequent injection with aggregated RGG. When animals, not pretreated with aggregate-freed RGG, were immunized with RGG and lipopolysaccharide (LPS), the immune response was greatly enhanced. The response of both pretreated low and high responders was substantially smaller than that of corresponding animals which were not given aggregate-freed RGG, prior to immunization. LPS revealed an inhibitory effect on low-responder mice of aggregate-freed RGG, which was not detected upon immunization with heat-aggregated RGG alone. The involvement of nonspecific suppressor cells and of B cell tolerance in low-responder mice is discussed.