Plaque-forming Cell Response Research Articles

Immunopharmacological actions of lumin (II): Effect of lumin administration in NZB X NZW (B/W) F1 mice

Lumin was administered at doses of 0.1 to 100 micrograms/kg for 5 months to NZB/W F1 mice as the model animal for studying systemic lupus erythematosus (SLE), a human autoimmune disease. The increase in anti-thymic autoantibody level was significantly inhibited at doses of 1 to 100 micrograms/kg. Also, the induction of suppressor T cells by concanavalin A was significantly promoted. In addition, recovery activity was significantly observed, over-coming the reduction in plaque-forming cell (PFC) response of anti-sheep erythrocytes at a dose of 100 micrograms/kg, as well as the reduction in PFC response of anti-trinitrophenylated-lipopolysaccharide at doses of 0.1 to 100 micrograms/kg. The above results prove that lumin exhibits an immunomodulating effect against immune disease in mice.

The influence of antibiotics on the immune system: immuno‐pharmokinetic investigations on the primary anti‐SRBC response in carp, Cyprinus carpio L., after oxytetracycline injection

Abstract. The present study was performed to obtain further details on the immuno‐modulating effects of oxytetracycline (OTC) in carp, Cyprinus carpio L. The immuno‐logical study was extended by investigating the pharmacokinetic behaviour of OTC. Oxytetracycline, at a dosage of 60 mg/kg, was administered five times intra‐peritoneally at 3‐day intervals; the first injection was given 1 day before immunization. The kinetics of the primary anti‐sheep red blood cell (SRBC) response, during a short‐term OTC treatment revealed that the kinetics of the plaque‐forming cell (PFC) response was not significantly affected by OTC nor its solvent. In contrast, the number of plaque‐forming cells was significantly decreased. The anti‐SRBC antibody production was delayed by 2–4 days in both experimental groups (OTC and solvent). However, within 12–14 days post‐immunization, the same antibody levels were detected as in the control group. High OTC plasma levels (>50 μg/ml) were detected about 10 h after a single i. p. injection. A mean plasma elimination half‐life (T1/2) of 34·5 h was calculated.

Alleviation of depressed immunity caused by restraint-stress, by the immunomodulator, lobenzarit disodium (disodium 4-chloro-2,2′-iminodibenzoate)

The immunomodulating effect of lobenzarit disodium (CCA, disodium 4-chloro-2,2′-iminodibenzoate), was examined in restraint-stressed mice which exhibited immunodeficient conditions mainly based on dysfunction of helper T cells. Primary direct plaque-forming cell (PFC) responses to T cell-dependent antigens such as sheep red blood cell (SRBC) and dinitrophenyl-keyhole limpet hemocyanin (DNP-KLH) were severely diminished by restraint-stress, whereas responses to T cell-independent antigens such as DNP-ficoll (DNP-Fic) and trinitrophenyl-lipopolysaccharide (TNP-LPS) were little reduced and sometimes were even increased. CCA, orally administered to mice at a dose of 50 mg/kg body weight, significantly restored anti-SRBC PFC response. Restraint-sterss also markedly decreased the proliferative response of splenic lymphocytes to concanavalin A (Con A), while it slightly augmented the response to lipopolysaccharide (LPS). CCA increased the response to Con A and suppressed the response to LPS. The numbers of T and B lymphocytes in the spleen were, however, reduced to the same degree by the stress and there was no striking difference in the susceptibility of these two subpopulations of splenic lymphocytes as analysed with fluorescein-activated cell sorter. CCA increased both T and B lymphocytes, but Lyt-1 + (helper/inducer T) cells recovered even more. It is noteworthy that in all these experiments CCA restored the weight of the thymus which is considered to be one of the organs most sensitive to stress. These results indicate that CCA promotes proliferation and differentiation of T cells, especially helper T cells, and restores immune functions depressed by restraint-stress.

Cortisone-induced recrudescence of Giardia lamblia infections in gerbils.

The course of infection with Giardia lamblia in gerbils was studied after oral inoculation with either cysts isolated from humans or cultured trophozoites. Primary infections occurred in 87% of exposed gerbils; cyst release peaked during the second and third weeks post-infection; cysts and trophozoites were observed for up to 49 and 87 days post-infection, respectively. Gerbils were refractory to challenge infection with infective human source cysts at 5 months post-infection. Overall, 50% of animals given subcutaneous injections of hydrocortisone acetate, during or following the late stages of their primary infection, showed recrudescence of infection as evidenced by passage of cysts; percentages showing recrudescence were: 80% at 50 days post-infection, 55% at 70 days post-infection, 27% at 7 months post-infection, and 50% (2 of 4) in gerbils challenged unsuccessfully at 5 months post-infection and subsequently injected with hydrocortisone acetate at 7 months post-infection. Both infected animals and uninfected controls injected with hydrocortisone acetate were immunocompromised as evidenced by significantly reduced plaque forming cell responses to sheep red blood cells. We conclude that infections with G. lamblia in gerbils can last longer than standard examinations for cysts would indicate, and that long-term immunity to this organism can be nonsterile.

Cellular immunity in patients with systemic juvenile rheumatoid arthritis

We studied the cellular immune responses in 10 patients with systemic form of juvenile rheumatoid arthritis. The numbers of peripheral T lymphocytes and their helper/inducer and cytotoxic/suppressor subpopulation were within normal levels. Activated T lymphocytes (DR +) were slightly increased but not at statistically significant levels. In contrast to the T cells, B lymphocytes were increased; both the percentage of B cells (B1 +) and the number of cells spontaneously secreting IgG, IgA, and IgM were increased. Stimulation of periphral mononuclear cells in vitro with pokeweed mitogen induced poor plaque-forming cell responses, which were partially improved upon removal of monocytes. The presence of concanavalin A in the cultures led to complete suppression. We conclude that patients with systemic JRA are characterized primarily by B-cell rather than T-cell abnormalities.

Isotypic pattern of the polyclonal B cell response during primary infection by Plasmodium chabaudi and in immune-protected mice.

The primary infection by P. chabaudi induces an increase of the numbers of splenic immunoglobulin (Ig)-secreting B cells in both athymic and euthymic BALB/c mice. The isotypic pattern of the polyclonal response is restricted only in euthymic mice where IgG2a, IgG2b and IgM plaque-forming cells (PFC) predominate. In immunized animals, protected against a parasite challenge, the isotypic pattern of splenic PFC is completely different, the IgG1 and IgM isotypes constituting the main part of the response. Reinoculation of immune-protected animals induces a PFC response which is dose dependent and accentuates the characteristic isotypic profile of the immune-protected mice.

Defective vaccine-induced resistance to Schistosoma mansoni in P strain mice. III. Specificity of the associated defect in cell-mediated immunity.

In contrast to many other strains, inbred P strain mice fail to develop significant levels of resistance to challenge Schistosoma mansoni infection as a result of prior vaccination with radiation-attenuated cercariae. In this study, the relationship between defects in resistance and development of cell-mediated immune reactivity was examined. Although splenocytes from immunized P mice demonstrated deficiencies in production of macrophage-activating lymphokine(s) in response to either antigenic or mitogenic stimulation, other aspects of T lymphocyte responsiveness including blastogenesis, production of interleukin 2, interleukin 3 and macrophage chemotactic factor, as well as helper cell function for secondary plaque-forming cell response to a T-dependent antigen and allospecific cytolytic T cell reactivity, appeared to be comparable with those of C57BL/6 mice, a strain that is protected by vaccination against S. mansoni. FACS comparison revealed no significant deficits in percentages of Thy-1+, Lyt-1+, or L3T4+ splenocytes in vaccinated P mice. The P-associated defect in production of macrophage-activating factor appeared to be at the level of the T cell rather than the antigen-presenting cell, because macrophages from P mice could reconstitute the lymphokine-producing capacity of T-enriched splenocytes from immunized, resistant (C57BL/6 X P) F1 or B10.P mice, whereas the converse was not true. These results indicate that vaccinated P mice have a selective defect in T cell function for production of macrophage-activating lymphokine, which is manifested as a failure to produce activated larvicidal macrophages at the site of specific antigen challenge in vivo and may be associated with the failure of this strain to become resistant to S. mansoni.

Eugenol modulation of the immune response in mice

The effect of eugenol on selected parameters of the immune response of C57BL/6 mice was studied. In a dose-response study, eugenol at high doses completely inhibited the plaque-forming cell responses of splenocytes to the T-dependent antigen (sheep erythrocytes) both in vitro and in vivo. In vitro, at concentrations higher than 1.0 mM, eugenol was found to be cytotoxic. In vivo, however, at very low doses there appeared to be a suppression of the plaque-forming cell response to sheep erythrocytes, reaching maximal suppression at 0.1 μmol eugenol/kg mouse body weight. This was followed by enhancement of the response, peaking at 0.25 μmol eugenol/kg mouse body weight. These changes correlated fairly well with body weight changes. Thymic weights appeared to remain unchanged for all doses except the 1.0 μmol eugenol/kg body weight, which was significantly (p<0.05) higher than the rest of the groups. Natural killer activity at all effector:target ratios (100:1, 50:1, 25:1) was significantly (p<0.05) enhanced at both 0.25 and 2.5 μmol eugenol/kg body weight. Overall, eugenol seems to have dose-dependent suppressive and enhancing effects on the immune response. These effects represent an atypical multiphasic response in which an inversed dose-response relationship is observed.

Immunomodulation by RS-2131, a new non-steroidal antiinflammatory drug

The immunomodulatory properties of RS-2131, 2-[4-(2-oxocyclohexylidenemethyl)phenyl]propionic acid, were studied using normal and autoimmune mice. RS-2131 suppressed the Arthus-type footpad reaction in BALB/c mice when administered orally at the time of immunization with sheep red blood cells, in contrast to indomethacin. This drug also suppressed the direct plaque-forming cell response against sheep erythrocytes in BALB/c mice. Under the same experimental conditions, the percentage ratio of Lyt 2+ cells to Thy 1.2+ cells were increased in the spleen cells. When used in non-immunized normal BALB/c mice, however, RS-2131 produced no effect on the ratio of Lyt 2+ cells to Thy 1.2+ cells in the spleen cells and on the percent distribution of T lymphocyte subsets. In MRL/Mp-lpr/lpr (MRL/lpr) mice, which have been known as a model of autoimmune disease, Lyt 2 cells in the thymus and the lymph node increased on oral treatment with RS-2131. These findings suggest that this drug may induce suppressor T cells only when the immune response in host animals is increased and then results in inhibition of immune responses in mice.

Cooperativity between B lymphocyte membrane molecules: independent ligand occupancy and cross-linking of antigen receptors and Fc gamma receptors down-regulates B lymphocyte function.

IgG antibody-antigen complexes bound to B lymphocyte Fc gamma receptor (Fc gamma R) but not surface immunoglobulin inhibit the antibody-forming cell response but not proliferation of these cells in response to F(ab')2 anti-mu and lymphokines. The role of B lymphocyte antigen receptors in B lymphocyte Fc gamma R-mediated inhibition was evaluated. With the use of several different antigen receptor-dependent plaque-forming cell (PFC) responses, it was found that inhibition occurred irrespective of the type of stimulatory signal used, or whether antigen receptors were bound by antibody or occupied by nominal antigen. The degree of inhibition appeared to be directly related to the extent of antigen receptor cross-linking. Maximal inhibition only occurred if both antigen receptors and Fc gamma R were occupied by their respective ligands simultaneously during the early hours of activation. In contrast, antigen receptor-independent PFC responses to the mitogen lipopolysaccharide (LPS) were not inhibited by complexes. However, the antigen-independent TNP-specific PFC response to LPS was inhibited by the combination of IgG antibody-antigen complexes and the hapten, but by neither alone. These results suggest that a down-regulatory signal is generated by functional cooperation between Fc gamma R and antigen receptors. Generation of this inhibiting signal could be mediated by the previously described physical interaction between these two receptors, and interactions between membrane receptors may be a general mechanism utilized by lymphocytes for the integration of multiple signals.

Human helper-T-cell function does not require T4 antigen expression

The relationship between immunoregulatory T-cell function and the expression of T-cell subset-specific differentiation antigens was examined using a phenotypically anomalous human T-cell line (TCL), termed H-1. H-1 cells were found to express T11, extremely high levels of T3, but no T4 nor T8 antigen. Despite their lack of T4 antigen expression, H-1 cells could be activated by coculture with pokeweed mitogen (PWM), anti-T3 antibody, or autologous B cells to provide potent help for B-cell differentiation into plaque-forming cells (PFC). In contrast, H-1 cells did not suppress the PFC response triggered by PWM-activated T4+ cells. These results demonstrate that the expression of the T-cell subclass-specific differentiation antigen, T4, is not required for a T cell to become activated and to implement the program for helper function. In addition, enhanced expression of T3 on the T4-, T8-, H-1 cell surface may reflect a compensatory upregulation of the T3/Ti receptor complex on T cells which are deficient in these nonpolymorphic associative recognition structures.

Regulation of the secondary in vitro antibody response by endogenous natural killer cells: kinetics, isotype preference, and non-identity with T suppressor cells.

Our previous studies had demonstrated that depletion of endogenous natural killer (NK) cells resulted in an augmented primary antibody response in vivo and in vitro. We have now examined the effect of NK cell depletion on the in vitro secondary response to antigen. Treatment of primed murine spleen cells with anti-NK-1.1 allo-antibody and complement before culture resulted in a significant increase in the magnitude of the antigen-specific plaque-forming cell (PFC) response. This treatment did not affect the proportions of Lyt-2+, L3T4+, or sIg+ cells in the population, however, indicating that the augmentation in PFC was not due to changes in the ratio of T to B cells. Removal of endogenous NK cells had a greater effect on the IgG (indirect) PFC response (100 to 200% increase) than on the IgM (direct) PFC response (25 to 50% increase). In contrast, removal of Lyt-2+ cells before culture affected the IgM and IgG responses similarly. Moreover, the kinetics of augmentation differed between cultures depleted of Lyt-2+ cells and those depleted of NK-1.1+ cells. NK cells appeared to act earlier in the response than did T suppressor cells. The NK-1.1+ cells involved in antibody regulation were not involved in the generation of the in vitro derived T suppressor cells. The conclusion that the regulation of the antibody response by NK-1.1+ cells is distinct from that involving T suppressor cells was confirmed in experiments in which removal of both regulatory cell populations resulted in an increase in PFC that was greater than in cultures depleted of either NK or T suppressor cells.

Stimulation of murine lymphocyte responses by cations

The capacity of the heavy cations Pb, Ni, and Zn to modulate murine in vitro lymphocyte responses was examined. Pb and Ni (100 μ M) were shown to enhance the in vitro plaque-forming cell (PFC) response to sheep red blood cells while 100 μ M Zn had inhibitory effects. Each metal was able to stimulate the proliferation of murine splenocytes as determined by [ 3H]thymidine incorporation and autoradiography. The enhancing effect of the metals on the PFC response was observed whether the results were expressed on a per culture or a per cell basis, indicating an actual increase in B-cell differentiation. Both the PFC response and the proliferative response were shown to be sensitive to the type of medium employed (M-199 gave optimum results) and to the presence or absence of 2-mercaptoethanol. As in autologous mixed-lymphocyte responses peak proliferation occurred after Day 5 in culture, was cell density dependent, and required the presence of both T cells and Ia + cells. Treatment of splenocytes with anti-Thy-1.2, anti-Lyt-1, or anti-L3T4 plus complement completely abrogated the proliferative response, indicating that a Lyt-1 +, Lyt-2 −, L3T4 + T-cell was required for the induction of proliferation. The data are consistent with the hypothesis that the metals are capable of modifying the immune response directed at self either by directly altering self constituents (class II) or by modulating the autologous T-cell response.

Dietary Fatty Acid Modulation of Murine B-Cell Responsiveness

These studies were designed to determine how dietary fat concentration and degree of saturation influence antibody response to a T-dependent antigen. In vivo fatty acid manipulation did not affect the total number of nucleated spleen cells or IgM or IgG-bearing cells. After primary immunization with sheep red blood cells (SRBC), the number of splenic IgM and IgG antigen specific plaque-forming cells (PFC) was lower in mice fed a diet containing a high level of polyunsaturated fat (PUF, safflower oil) than in mice fed a diet containing minimal essential fatty acids (EFA, 0.5% corn oil). Mice fed a high level of saturated fat (SF, coconut oil) exhibited greater IgM PFC responses than the control. After secondary immunization, the number of IgG-producing cells followed a similar response pattern. These differences were also reflected in the serum anti-SRBC IgM and IgG levels as determined by solid-phase radioimmunoassay. Although the level of linoleic acid in whole lymphocytes has been previously reported to change in direct relation to the serum fatty acid level, no differences in the fatty acid composition of isolated lymphocyte plasmalemma were observed. Thus, changes in the number of antibody-forming cells appear to be inversely related to the levels of linoleic acid in other cell compartments. Fluorescence polarization measurements of the lipopholic probe, 1, 6-diphenyl-1,3,5-hexatriene (DPH), in the purified lymphocyte plasmalemma from mice fed a diet high in PUF indicated an increase in mobility. Polarization values for lymphocyte plasmalemmas from mice receiving diets high in SF were not significantly different from the controls. We conclude that dietary fat modulation of splenic B-cell responses was manifested through changes in the number of cells producing antibody and level of antibody produced, not through total splenic B-cell numbers. This B-cell response can be modified, depending upon the fatty acids available to the cell, and may be related to differential effects upon plasma membrane structure.

Abnormal B-cell function in hemophiliacs treated with cryoprecipitate and factor VIII and IX concentrates

B-Cell function was evaluated in a group of 43 patients with factor VIII or factor IX deficiency. Thirty had been treated primarily with cryoprecipitate and 13 with concentrates of factor VIII or IX. Serum immunoglobulin G levels were found to be diffusely elevated; however, the absolute number of mature B cells in peripheral blood was normal. B-Cell function as measured by testing mitogen-induced proliferation and immunoglobulin secretion by plaque-forming cells (PFC) in vitro was reduced. Coculture experiments suggested that these abnormal B-cell responses might be secondary to increased suppressor T-cell activity, which was found more frequently in patients seropositive for antibody to lymphadenopathy-associated virus. Both seronegative and seropositive patients had reduced responses in the proliferative and PFC assays, but the lowest PFC responses occurred in the seropositive group.

Suppression of immune responses to sheep erythrocytes by the lymphokine soluble immune response suppressor (SIRS) in vivo.

Soluble immune response suppressor (SIRS) is a protein produced by activated suppressor T lymphocytes which inhibits division by tumor cells and plaque-forming cell (PFC) responses in vitro. Although this lymphokine has been fairly well characterized in vitro, little is known about its effects in vivo. Purified murine SIRS, 10(3) to 10(4) U injected i.p., suppressed murine PFC responses to sheep erythrocytes (SRBC) in vivo. Suppression occurred when SIRS was injected into mice 5 days before assay, and also occurred when SIRS activated with 10(-6) M H2O2 was injected 24 hr before assay. These kinetics are similar to those observed in tissue culture, where suppression of PFC responses requires the addition of SIRS 4 to 5 days before assay unless SIRS is activated to SIRSox by H2O2. Levamisole, an inhibitor of SIRS-mediated suppression in vitro, also blocked suppression by SIRS in vivo. Delayed-type hypersensitivity reaction to footpad injection of SRBC was also inhibited by SIRS. Suppression of PFC responses by recombinant immune interferon (IFN-gamma), which activates lymphocytes to produce SIRS in vitro, was blocked by injection of levamisole or monoclonal anti-SIRS antibodies. These results show that SIRS suppresses immune responses in vivo, and suggest that suppression of PFC responses by IFN-gamma may be largely mediated by SIRS. These findings indicate that SIRS could contribute to the development of suppressed immunity in vivo.

Role of self carriers in the immune response and tolerance. X. A lymphoid dendritic-like tumor, P388AD.2, acts as a novel immunogenic carrier for hapten.

Hapten-modified spleen cells, peritoneal exudate cells, and certain lymphoid tumors preferentially induce specific tolerance after i.v. administration. In contrast to these tolerogenic carrier cells, we found that a haptenated lymphoid dendritic-like tumor, P388AD.2, acts as a potent immunogen after i.v. injection. The immunogenicity of P388AD.2 was analyzed by measuring the specific augmentation of plaque-forming cell (PFC) responses when spleen cells from mice previously injected with haptenated tumor cells were challenged in vitro with thymus-independent antigens. Optimal immunization was found to be dependent on cell dose and hapten concentrations. Further studies indicated that P388AD.2 elicited a response which was T cell-dependent and which involved both the so-called Lyb-3,5,7- and Lyb-3,5,7+ B cell populations. Injection of haptenated tumor into different mouse strains suggested that H-2 compatibility was required to prime B cells in vivo, although significant augmentation could also be achieved in allogeneic C57B1/6J mice. The enhanced PFC responses elicited in H-2b mice could not be explained by allo-recognition of class I or II MHC determinants. In toto, these results suggest that P388AD.2 acts as a unique accessory cell for the presentation of hapten-modified self.

Selective immunomodulation by the antineoplastic agent mitoxantrone. I. Suppression of B lymphocyte function.

Novantrone mitoxantrone, an antineoplastic agent with antiproliferative properties, is under investigation as an immunomodulating agent. The impact of mitoxantrone treatment on B lymphocyte reactivity is presented here. Administered i.p. in H2O at a dose of 0.5 mg/kg, daily for 14 days, mitoxantrone abrogated both the in vivo antibody response (to ovalbumin) and the in vitro plaque-forming cell (PFC) response (to SRC). In addition to the effects on thymus-dependent reactivity, PFC responses to the thymus-independent antigens TNP-LPS and TNP-Ficoll were also inhibited when tested in vivo or in vitro. B cells were identified as a target for the suppressive activity of mitoxantrone by using T cell-replacing factor to reconstitute the in vitro anti-SRC PFC response of a T lymphocyte-depleted spleen cell preparation. LPS-induced B cell mitogenesis was largely inhibited by mitoxantrone treatment. However, depletion of Sephadex G-10-adherent cells significantly restored the proliferative response. Flow cytometric analysis revealed a dramatic decrease in splenic B lymphocyte content. Therefore, mitoxantrone exerted a potent suppressive influence on the humoral immune system through a direct reduction in B cell number augmented by macrophage-mediated inhibition of B cell proliferation.

Aberrancy in immunogenicity and cell-surface expression of H-2 antigens on erythrocytes.

Immunogenicity for T cell-independent B-cell response assessed by splenic plaque-forming cell (PFC) response and cell-surface expression measured by laser flow cytometry of various class I H-2 antigens on mouse red blood cells (RBC) were compared. It was found that the order of magnitude of both immunogenicity and cell-surface expression on RBC is H-2Dd much greater than H-2Db greater than H-2Kd, H-2Kb. Furthermore, H-2d public antigens and H-2Ld antigens were neither immunogenic nor easily demonstrable on RBC. These findings contrasted with poor immunogenicity for PFC response (Nakashima et al. 1982, 1983) and proportionally strong expression of H-2 antigens on lymphoid cells. Immunogenicity and cell-surface expression of H-2Dd antigen on RBC were not shown to be controlled by the action of genes outside H-2D. It was therefore suggested that a number of H-2 antigens, including H-2Kd private, H-2Kb private, and H-2d public specificities are at least functionally defective on RBC. This is possibly due to the structural characteristics of the antigens. Since immunogenicity and cell-surface expression were in parallel, the expression of H-2 antigens on RBC must be dictated by a subset of B cells whose activity was assessed by PFC response. This finding supports the view that the H-2 molecules display a new category of activity which is different from their ability to activate T cells and depends on their expression on RBC.

Modulation of cellular immune responses in mice with disseminated histoplasmosis by recombinant interleukin-2.

Depression of the cellular immune responses in mice with disseminated histoplasmosis is associated with deficient production of interleukin-2 (IL-2) by splenocytes. Therefore, we examined whether a highly purified preparation of IL-2, recombinant human IL-2 (rIL-2), could modify the cellular immune responses in infected mice and whether this lymphokine could alter the severity of histoplasmosis in animals. Exogenous rIL-2, at concentrations of up to 1,000 U/ml, failed to augment the proliferative responses to concanavalin A by unfractionated splenocytes or splenic T cells from mice infected for 1 week. In addition, rIL-2 did not modulate the plaque-forming cell response to sheep erythrocytes by splenocytes from these same mice. However, at week 3, rIL-2 in concentrations ranging from 10 to 1,000 U/ml considerably augmented the proliferative response to concanavalin A and plaque-forming cell response to sheep erythrocytes by splenocytes from infected mice. Kinetics studies demonstrated that rIL-2 exerted maximal immunoregulatory activity when added on day 0 or 1 to cultures of splenocytes. In vivo administration of rIL-2, 200 to 20,000 U/day, for 10 days to normal and 3-week-infected mice did not alter the proliferative activity of splenocytes to concanavalin A; 200,000 U of rIL-2 per day actually depressed the proliferative responses of splenocytes from normal and infected mice. In vivo, rIL-2 did not modify delayed-type hypersensitivity responses to sheep erythrocytes or to histoplasmin by normal and infected mice. Moreover, treatment with rIL-2 in vivo did not reduce the number of Histoplasma CFU in spleens of mice. Thus, despite the immunoenhancing effect of rIL-2 in vitro, this lymphokine failed to exert similar effects in vivo.