Bronchoalveolar cells from sarcoid patients demonstrate enhanced antigen presentation

The recognition of foreign antigens by T lymphocytes in association with lung antigen-presenting cells may be critical in the initiation of the mononuclear alveolitis and granuloma formation of pulmonary sarcoidosis. However, it has been shown that bronchoalveolar cells (BAC) from normal volunteers function poorly as antigen-presenting cells. Therefore, the ability of sarcoid BAC to serve as accessory cells for antigen-dependent autologous T cell proliferation, as measured by tritiated thymidine uptake, was compared with that of normal BAC. Although irradiated sarcoid BAC supported antigen-induced T cell proliferation, normal BAC did so poorly (p less than 0.005). Because it has been shown that sarcoid BAC produce more interleukin 1 (IL 1) than normal BAC, it was considered that the enhancement of antigen-induced proliferative responses could result from an increased amount of IL 1, and that contaminating monocytes in the peripheral blood T cell preparations displayed the antigen for T cell recognition. Therefore, it was necessary to establish that antigen-induced T cell responses required HLA-D region compatibility between the sarcoid BAC and T lymphocytes. BAC from sarcoid patients stimulated antigen-specific proliferation in T cells lines matched for at least one HLA-D-region antigen, but failed to stimulate T cell lines that were unmatched for both antigens. This finding indicates that cells in bronchoalveolar lavage fluids from sarcoid patients were fully capable of acting as antigen-presenting cells. The identification of antigen-presenting cells in the lungs of patients with sarcoidosis together with the previous findings of activated T cells, enhanced IL 1 production, and spontaneous interleukin 2 release in sarcoid patients is compatible with the hypothesis that local cell-mediated immunity is involved in the pathogenesis of pulmonary sarcoidosis.     

Interleukin 2 administered in vivo induces the growth of cultured T cells in vivo

The capacity of exogenous IL 2 to induce the growth of antigen-activated T lymphocytes in vivo was evaluated. The in vivo growth of adoptively transferred T lymphocytes that had been previously cultured long-term with IL 2 was initially examined, because in vitro such T cells are exquisitely dependent upon exogenous IL 2 for proliferation and survival. Daily administration of IL 2 in vivo, beginning on the day of cell transfer, induced these IL 2-dependent long-term cultured T lymphocytes to proliferate in vivo, and the magnitude of in vivo growth was proportional to the dose of IL 2 administered. The capacity of IL 2 to induce the in vivo growth of antigen-activated T cells not previously exposed in vitro to exogenous IL 2 was similarly studied. T lymphocytes from the spleens of immune mice, activated by 5-day culture with tumor antigen before transfer, survived poorly in vivo when injected with antigen alone, but demonstrated marked proliferation in vivo in response to antigen and exogenous IL 2. By contrast, immune spleen cells transferred with antigen, but without prior culture, proliferated without supplementary exogenous IL 2. Moreover, the growth of noncultured donor T cells was not augmented by the administration of exogenous IL 2, implying that noncultured spleen cells immune to tumor antigens can produce sufficient amounts of endogenous IL 2 in vivo to sustain maximal T cell growth over the time period examined. Importantly, the ability of exogenous IL 2 to induce donor T cell growth in vivo correlated with its ability to function in vivo to augment the anti-tumor efficacy of specifically immune donor T cells in models for the adoptive therapy of disseminated antigenic murine leukemia. Thus, the current studies highlight the potential of exogenous IL 2 to induce T cell growth in vivo and suggest that the administration of IL 2 in vivo may be useful for augmenting T cell responses that are relatively deficient in the production of endogenous IL 2.     

Histidine-rich glycoprotein blocks T cell rosette formation and modulates both T cell activation and immunoregulation

Histidine-rich glycoprotein (HRGP) is a plasma and platelet protein with undefined function in vivo. It has been reported to inhibit rosette formation between murine T cells and erythrocytes. We have shown that HRGP binds specifically to human T lymphocytes but not sheep erythrocytes and have demonstrated a 56-kDa HRGP-binding protein on the T cell surface which is distinct from the CD2 sheep erythrocyte receptor. We have now investigated whether HRGP can inhibit human T cell-sheep erythrocyte rosette formation and whether HRGP can modulate T cell activation. HRGP at physiologic concentrations specifically inhibited rosette formation between human T lymphocytes and sheep erythrocytes. HRGP suppressed proliferation of antigen receptor (CD3)-triggered T cells induced by interleukin 2; this suppression was specifically reversed by prior incubation of HRGP with affinity-purified anti-HRGP IgG. Addition of HRGP 12-24 h after CD3 triggering no longer suppressed T cell proliferation, suggesting HRGP suppressed T cell division by interfering with one or more early events in the process of T cell activation. Human serum (containing 100-150 micrograms/ml HRGP) was also capable of suppressing T cell proliferation; serum which had been immunodepleted of HRGP no longer inhibited T cell proliferation. Furthermore, HRGP inhibited interleukin 2 receptor expression on activated T cells, causing decreased T cell interferon-gamma release and altered T cell-dependent inhibition of erythropoiesis. HRGP is thus capable of modulating T cell activation and T cell immunoregulation; HRGP may function as a natural suppressive regulator of human T lymphocyte activation.     

Interleukin 2 receptors are expressed by alveolar macrophages during pulmonary sarcoidosis and are inducible by lymphokine treatment of normal human lung macrophages, blood monocytes, and monocyte cell lines

Expression of receptors for IL 2 was believed initially to be restricted to T cells after their activation by IL 1 and antigen. However, recently IL 2 receptors (IL 2R) were demonstrated on activated B cells by using an anti-IL 2R monoclonal antibody (anti-Tac). In this study, we examined the capacity of cultured human alveolar macrophages, blood monocytes, and myelomonocytic (HL-60) or monoblast (U937) cell lines to bind three different anti-IL 2R monoclonal antibodies before or after stimulation with the monocyte-activating agents IFN-gamma, LPS, phorbol ester, or lymphokine-containing conditioned medium. For each of the four cell populations examined, resting unstimulated cells bound little or no anti-IL 2R antibody, as shown independently by quantitative cell binding assay and by immunoperoxidase labeling. By contrast, incubation with recombinant IFN-gamma, conditioned medium, or to a lesser extent, native or recombinant IL 2 itself, resulted in a significant enhancement of anti-IL 2 receptor monoclonal antibody binding by all four populations, whereas LPS, PMA, or IL 1 had no effect. In addition, membrane binding of anti-Tac antibody, similar to that seen after stimulation of normal lung macrophages with IFN-gamma, was detected by using macrophages obtained by bronchoalveolar lavage of five patients with active pulmonary sarcoidosis. These findings are consistent with the expression of a functional IL 2R on activated cells of the monocyte lineage, since anti-Tac binding to IFN-gamma-treated HL-60 cells was inhibited by addition of excess IL-2; specific binding of anti-IL 2 monoclonal antibodies was detected in the presence of exogenous IL 2; and a 50 to 55 kD molecule was immunoprecipitated from both activated lung macrophages and T lymphoblasts by using anti-Tac antibody. We conclude that human mononuclear phagocytes can be induced by lymphokines to express IL 2R, and that such IL 2R+ macrophages can be detected in vivo during inflammation.     

Sarcoidosis is not associated with a generalized defect in T cell suppressor function

In pulmonary sarcoidosis, the marked expansion of CD4+ (helper/inducer) T cells in the alveolar structures of the lung is maintained by local IL-2 release by activated CD4+ HLA-DR+ T cells without concomitant expansion and activation of CD8+ (suppressor/cytotoxic) T cells, suggesting that sarcoid may be associated with a generalized abnormality of CD8+ T cells. Consistent with this concept, evaluation of the expression of the IL-2R on fresh lung T cells from individuals with active sarcoidosis demonstrated that 7 +/- 1% of sarcoid lung CD4+ T cells are spontaneously expressing the IL-2R compared with only 1 +/- 1% lung CD8+ T cells (p less than 0.01). However, stimulation of purified sarcoid blood CD8+ T cells with the anti-T3/TCR complex mAb OKT3 was followed by the normal expression of IL-2R (p greater than 0.1) and proliferation (p greater than 0.1). In addition, lung sarcoid CD8+ T cells responded to OKT3 similarly to normal lung CD8+ T cells and to autologous blood CD8+ T cells as regards expression of IL-2R (p greater than 0.1) and proliferation (p greater than 0.1). Finally, using CD4+ cells activated with allogenic Ag to induce, in coculture, fresh autologous CD8+ cells to suppress proliferation of fresh autologous CD4+ cells to the same Ag, sarcoid CD8+ T cells suppressed CD4+ cell proliferation in a normal fashion (p greater than 0.1). These results demonstrate that sarcoid CD8+ (suppressor/cytotoxic) T cells are competent to respond to a proliferation signal normally and can be induced to normally suppress CD4+ T cell proliferation to Ag, suggesting that the expansion of activated CD4+ T cells in pulmonary sarcoidosis is not due to a generalized abnormality of CD8+ T cells or of their suppressor T cell function.     

Polymorphisms at position -308 in the promoter region of the TNF-alpha and in the first intron of the TNF-beta genes and spontaneous and lipopolysaccharide-induced TNF-alpha release in sarcoidosis.

TNF-alpha is a potent pro-inflammatory cytokine. Previous studies have proved that biallelic polymorphisms in the TNF-alpha (-308, TNFA) and TNF-beta genes (intron 1, TNFB) influence TNF-alpha production. In sarcoidosis, a chronic granulomatous disease, as a result of an unknown in vivo activation bronchoalveolar lavage (BAL) cells release high amounts of TNF-alpha, spontaneously and after in vitro stimulation. Thus, sarcoidosis could serve as a model to test the in vivo effect of TNF gene polymorphisms. We determined the TNFA and TNFB polymorphisms of 44 patients with sarcoidosis and found the following allele frequencies: 0.80, 0.20, 0.38 and 0.62 for TNFA1, TNFA2, TNFB1 and TNFB2, respectively. To examine the in vivo effect of the named polymorphisms on the TNF-alpha production, the spontaneous and LPS-induced TNF-alpha release of BAL cells and peripheral blood mononuclear cells were also determined in patients with sarcoidosis. Statistical analysis did not reveal any significant difference between sarcoidosis patients with different genotypes. The results show that TNFA and TNFB polymorphisms do not determine the level of TNF-alpha release of mononuclear cells activated during the course of sarcoid inflammation.     

The role of IL 1 in the antigen-specific activation of murine class II-restricted T lymphocyte clones

The role of IL 1 in the antigen-specific activation of class II-restricted T lymphocytes was examined by using a model system consisting of cloned WEHI 5 B lymphoma accessory cells and class II-restricted, soluble antigen- or alloantigen-reactive T cell clones. The addition of exogenous recombinant IL 1 to the T cell cultures resulted in a significant enhancement of the antigen-specific T cell proliferation response, but at best, only small increases in IL 2 release. Goat IgG anti-IL 1 antibodies were added to the T cell cultures to assess their effect on T cell activation. The IL 1 enhancement of the T cell proliferation response was inhibited by the anti-IL 1 antibodies in a dose-dependent manner. In contrast, only modest levels (10 to 25%) of proliferation inhibition were observed in T cell cultures containing either WEHI 5 or splenocyte accessory cells but no exogenous IL 1. When the anti-IL 1 antibodies were added to primary mixed lymphocyte cultures stimulated by WEHI 5 cells in the absence of exogenous IL 1, no significant inhibition of proliferation was observed. A small but statistically significant proliferation inhibition was observed when anti-IL 1 antibodies were added to mixed lymphocyte reaction cultures stimulated by splenocytes. Two-color cytofluorometric analysis of the effects of IL 1 on antigen-activated T cell clones demonstrated that under suboptimal stimulation conditions, IL 1 stimulated a small but significant increase in the number of T cells bearing IL 2 receptors. In the presence of optimal numbers of WEHI 5 accessory cells, IL 1 enhanced T cell proliferation in the absence of a detectable increase in the number of T cells bearing IL 2 receptors, the number of IL 2 receptors per T cell, or the levels of IL 2 released. Finally, exogenous IL 1 can be added as late as 18 to 24 hr after culture initiation without significantly reducing its ability to enhance the T cell proliferation response. These data indicate that IL 1 has pleiotropic effects on murine T lymphocytes and can function to enhance T cell activation at multiple points during the activation sequence.     

Exposure of rats to a 50-Hz, 100 muTesla magnetic field does not affect the ex vivo production of interleukins by activated T or B lymphocytes.

Two separate, independent experiments were conducted to evaluate the effects of exposure of rats to a 50-Hz linearly polarized, 100 microT magnetic field (MF) on the ex vivo production of interleukins (ILs) by mitogen-stimulated splenic lymphocytes. IL-1 and IL-2 were determined by proliferation assays, using IL-dependent murine T cell lines. In the first experiment, female Sprague-Dawley rats were treated with 7,12-dimethylbenz[a]anthracene (DMBA] at a dose of 20 mg per rat (four weekly gavage doses of 5 mg), and were either MF-exposed or sham-exposed for 14 weeks. This experimental protocol has previously been shown to result in a significant increase in breast cancer growth in response to MF exposure. Furthermore, MF exposure at 50-100 microT for 3 months was recently found to induce a suppressed ex vivo proliferation of splenic T cells in response to mitogen stimulation, which could be a result of reduced IL production of spleen lymphocytes. However, the present experiments failed to demonstrate any significant difference between MF- and sham-exposed groups in production of IL-1 by mitogen-activated splenic B cells. In a second experiment, shorter MF exposure periods were studied with respect to IL production from mitogen-stimulated B and T cells. Groups of rats were MF- or sham-exposed for 1 day, 1 week, or 2 weeks, followed by preparation and activation of spleen lymphocytes. No significant difference in IL-1 or IL-2 production from stimulated B or T cells was seen. The data indicate that in vivo MF exposure of rats does not affect the ex vivo IL production of B or T lymphocytes, suggesting that the recently reported changes in T cell proliferation in response to MF exposure may not be mediated via alterations in B or T cell IL production.     

Activation of virus specific CTL clones: antigen-dependent regulation of interleukin 2 receptor expression

Interleukin 2 (IL 2) is a lymphocyte-specific growth hormone, whose effect on lymphocyte proliferation is exerted through a cell surface receptor expressed on activated lymphocytes. In this report we have used monoclonal antibodies directed to the murine IL 2 receptor to examine the regulation of the IL 2 receptor expression on cloned populations of influenza virus-specific CTL. The CTL clones, which are dependent on both specific antigenic stimulation and exogenous IL 2 for continuous in vitro propagation, express high levels of the IL 2 receptor shortly after antigenic stimulation (day 2 or 3). Over the next 5 to 8 days of in vitro cultivation in IL 2-containing medium, these cloned CTL cells express decreasing levels of IL 2 receptor. Concomitant with this fall in IL 2 receptor expression, the cells become refractory to the IL 2 proliferative stimulus. The cloned cells remain refractory to IL 2 until specifically stimulated by antigen, which induces high levels of the IL 2 receptor on the cells and renders the cells sensitive to IL 2 once again. These results support the concept that IL 2 receptor expression on activated T lymphocytes is transitory and that receptor expression is endogenously regulated in the activated T lymphocytes. These results also suggest that antigen plays a primary role in regulating T lymphocyte proliferation by maintaining IL 2 receptor levels.     

A 44 kilodalton cell surface homodimer regulates interleukin 2 production by activated human T lymphocytes

We previously described a cell surface antigen, termed Tp44, detected by monoclonal antibody 9.3 on approximately 80% of mature human T lymphocytes. Analysis by SDS-polyacrylamide gel electrophoresis and isoelectric focusing demonstrated that this antigen consists of two identical 44 kilodalton glycopeptides that form a disulfide-linked homodimer. Competitive binding experiments showed that antibody 9.3 and an anti-CD3 antibody (64.1) recognize distinct antigenic determinants; furthermore, the binding of antibody 9.3 was unaffected by prior modulation of CD3. Thus, Tp44 has no detectable cell surface association with CD3. By itself, antibody 9.3 had no detectable effect on either IL 2 receptor expression or IL 2 release, and did not cause T cell proliferation even when monocytes were present and exogenous IL 2 was provided, indicating that binding of antibody 9.3 does not provide a primary signal for T cell activation. However, the proliferative responses of T lymphocytes activated by phytohemagglutinin, concanavalin A, or an anti-CD3 monoclonal antibody were strikingly enhanced in the presence of antibody 9.3, an effect associated with increased IL 2 receptor expression and increased IL 2 secretion. Antibody 9.3 enabled anti-CD3-Sepharose-activated T cells and anti-CD3 antibody-activated Jurkat cells to release IL 2 in the absence of monocytes. Fab fragments of antibody 9.3 had no effect on anti-CD3-induced IL 2 release by Jurkat cells, whereas F(ab')2 fragments had activity comparable to that of unmodified antibody, indicating that bivalent binding of Tp44 molecules is required for IL 2 secretion. Together, these results suggest that TP44 may function as a receptor for accessory signals in the activation of T cells.     

Differential effect of anti-beta 2-microglobulin on IL 2 production and IL 2 receptor expression in the primary mixed lymphocyte culture reaction

The mechanism of inhibition of the proliferative response in primary mixed lymphocyte culture (1 degree MLC) by antibodies to beta 2-microglobulin (beta 2m) was investigated. It is demonstrated that anti-beta 2m antibodies inhibit the production of interleukin 2 (IL 2). In contrast, the expression of IL 2 receptor is not affected by anti-beta 2m. The addition of purified exogenous IL 2 to the antibody-treated 1 degree MLC can completely restore the proliferative response, indicating that anti-beta 2m does not interfere with IL 2 binding to its receptor. Similarly, anti-beta 2m does not interfere with the capacity of IL 2-dependent T cell lines or T cell clones to respond to exogenous IL 2. The inhibition of cell proliferation and IL 2 production by anti-beta 2m is maximal when the antibody is added at the beginning of 1 degree MLC culture, and no effect of anti-beta 2m is seen when added after 3 days of culture. Anti-beta 2m has no effect on mitogen-induced cell proliferation and IL 2 production. Anti-beta 2m acts on the responder cell population, as demonstrated in experiments in which responder cells or stimulator cells are treated separately with the antibody. The expression of HLA-class II antigens (i.e., HLA-DR and DQ (DC) on the T cells activated on 1 degree MLC is not affected by anti-beta 2m. These studies indicate that the HLA-beta 2m class I antigen complex plays a role in T lymphocyte activation via release of IL 2, and suggest the existence of different mechanisms for activation of IL 2 producers and IL 2 responders in 1 degree MLC.     

Two-dimensional analysis of interleukin 2-regulated tyrosine kinase activation mediated by the p70-75 beta subunit of the interleukin 2 receptor

The proliferation of activated T lymphocytes is dependent on the interaction of the polypeptide growth factor interleukin 2 (IL 2) with its heterodimeric receptor, which consists of a p55 alpha subunit and a p70-75 beta subunit. Previously, it was shown that IL 2 stimulates rapid serine phosphorylation of several membrane and cytysolic proteins. Here, using anti-phosphotyrosine antibodies to purify phosphotyrosyl proteins and two-dimensional gel analysis, we show that IL 2 stimulates rapid tyrosine phosphorylation of a variety of cellular proteins, including pp180, pp92, and pp42 in activated human T lymphocytes. In addition, we have examined IL 2-induced tyrosine phosphorylation in the human cell line YT2C2 which expresses mostly the beta subunit of the IL 2 receptor and the gibbon cell line MLA-144 which expresses only the beta subunit. In both of these cell lines, IL 2 induced tyrosine phosphorylation of the same proteins phosphorylated in normal human T lymphocytes in response to IL 2. We conclude that the beta subunit is sufficient to induce tyrosine phosphorylation of the normal cellular target substrates involved in signal transduction.     

Helper B cells promote cytotoxic T cell survival and proliferation independently of antigen presentation through CD27/CD70 interactions

CD8-expressing cytotoxic T cell (CTL) interactions with APCs and helper T cells determine their function and ability to survive. In this study, we describe a novel interaction independent of Ag presentation between activated CTLs and bystander CD19-expressing B lymphocytes. Ag-stimulated CTLs serially engage autologous B lymphocytes through CD27/CD70 contact that promotes their survival and proliferation. Moreover, these interactions induce the release of proinflammatory cytokines that follows two general patterns: 1) an epitope-dependent enhancement of cytokine release, and 2) a previously undiscovered coordinate release of cytokines independent of epitope exposure. The latter includes chemoattractants targeting activated T cells. As a result, activated T cells are attracted to B cells, which exert a "helper" role in lymphatic organs or in areas of inflammation. This observation provides a mechanistic explanation to previously reported experimental observations suggesting that B cells are required for T cell priming in vivo.     

The interleukin 2 secretion defect in vitro in systemic lupus erythematosus is reversible in rested cultured T cells

Interleukin 2 (IL 2) secretion in response to mitogenic stimulation in vitro is strongly reduced in circulating T lymphocytes from patients with SLE. It is still not clear how this abnormality relates to the B cell hyperactivity in the disease. Some investigators proposed that an intrinsic T helper cell defect could lead to suppressor cell dysfunction and autoimmunity. Others have found that in fact increased suppressor cell activity can cause IL 2 hyposecretion. In the present study we report that the IL 2 secretion in response to PHA plus PMA by T cells from patients with SLE, which initially was decreased by a factor of 10 as compared with the IL 2 secretion in blood donor T cells, was restored when the T cells were rested for 2 to 3 days in culture before stimulation. IL 2 hyposecretion in SLE T cells and the kinetics of normalization in culture were not changed by the addition of normal adherent cells during the stimulation with PHA/PMA, occurred in the absence of significant cell death or proliferation or change of the T4:T8 cell ratio during the resting culture, were not due to a maturation of immature T6-positive cells (less than 1.5% T6 cells in SLE T cells), and also occurred in T8-depleted T4 cells alone. Furthermore, a normalization of IL 2 secretion took place in the presence of either SLE serum or normal serum, and the addition of fresh autologous T cells to 3-day-cultured SLE T cells did not cause suppression of IL 2 secretion. These data show that some rapidly reversible defect occurs in circulating T helper cells in SLE. That this could reflect an exhaustion of T helper cells that have been activated in vivo is discussed.     

Accessory function and interleukin 1 production by human leukemic cell lines

Highly purified T lymphocytes do not proliferate in response to mitogens, unless adherent HLA-DR-positive monocytes are added to the culture. This accessory function (AF) of monocytes requires the release of interleukin 1 (IL 1). Cells from three human leukemic cell lines, K562, HL60, and U937, could very efficiently replace monocytes in a 72-hr mitogen-induced T cell proliferation assay. The AF was clearly related to precise maturational stages of these cells; the hematopoietic precursor K562 cells spontaneously exerted high AF, but lost this property when treated with differentiation inducers. On the contrary, the promyelocytic HL60 cells and the "histiocytic" U937 cells exhibited no spontaneous AF, but acquired this property when induced to differentiate along the granulocytic and/or monocytic pathway. Three leukemic cells could not only stimulate T cells to proliferate and produce IL 2 in the presence of mitogens, but also under appropriate culture conditions these cells could produce IL 1, which could not be distinguished from normal human monocyte derived IL 1 by gel filtration and isoelectric focusing. Moreover, analysis of phenotypic markers revealed that AF and production of IL 1 could be demonstrated in different cell types and therefore are not restricted to the monocytic lineage. No HLA-DR antigen could be detected on K562 and HL60 cells. Thus, the expression of the DR antigens is not required for AF and IL 1 production in response to mitogens. These three human leukemic cell lines will provide convenient sources of human IL 1.     

Systemic administration of recombinant interleukin 2 stimulates in vivo lymphoid cell proliferation in tissues

Recent work in our laboratory has demonstrated that the repeated injections of high doses of recombinant interleukin 2 (IL 2) can dramatically reduce the number of established pulmonary and hepatic metastases and the growth of intradermal tumors in a variety of murine tumor models. We have thus undertaken studies to define the mechanisms underlying these in vivo effects of IL 2. Using an in vivo DNA-labeling technique in which we employed 5-[125I]iodo-2'-deoxyuridine (125IUdR), we examined the in vivo cell proliferation in the tissues of mice treated with IL 2. A proliferation index (PI) was calculated by dividing the raw counts per minute (cpm) of tissues in IL 2-treated mice by the cpm in corresponding tissues of control animals. At an IL 2 dose of 6000 U given i.p. three times a day, the highest 125IUdR incorporation was seen in the lungs, liver, spleen, kidneys, and mesenteric lymph nodes (PI = 6.9, 6.9, 5.1, 7.1, 24.6, respectively, at 5 days). The amount of lymphoid proliferation in these organs was a direct function of the dose of IL 2 administered. Other tissues including thymus, intestines, skin, and hind limb showed no significant increase in 125IUdR uptake even after host treatment with the highest doses of IL 2. Blood and brain demonstrated intermediate incorporation of the radiolabel. Preirradiation of the host largely eliminated the proliferative response to IL 2. Histologic studies of normal and irradiated mice receiving IL 2 corroborated the result of the 125IUdR findings. In normal IL 2-treated mice, large collections of activated lymphoid cells were seen, most prominently in the lungs, liver, and kidneys, whereas markedly decreased lymphoid proliferation was evident histologically in preirradiated mice. A fluorescein-labeled monoclonal antibody directed against the Thy-1.2 surface determinant was used to identify these dividing cells in frozen tissue sections as T lymphoid cells. Activated lymphocytes isolated from the lungs, liver, spleen, and mesenteric lymph nodes of IL 2-treated mice demonstrated significant lysis of a fresh murine sarcoma target in short-term 51Cr-release assays. These studies demonstrate that the systemic administration of recombinant IL 2 causes in vivo activation and proliferation of host lymphoid cells and has important implications for the adoptive immunotherapy of tumors.