Cyclic AMP inhibits macrophage suppressor function and enhances lymphocyte proliferation.

The effects of increasing the level of cyclic AMP (cAMP) activity on lymphocyte proliferation in the rat mixed lymphocyte reaction (MLR) were investigated. Dibutyryl cAMP (dbcAMP) and prostaglandin E2 (PGE2) produced a dose-dependent reduction in proliferation in the lymph node (LN) MLR, but produced a substantial increase in proliferation in the spleen MLR at the lower concentrations used (10(-5)-10(-4) M dbcAMP; 10(-7)-10(-6) M PGE2). Enhancement of proliferation was dependent on the presence of macrophages and was probably due to inhibition of macrophage activation. This was based on the following findings: (1) spleen MLR proliferation was lower than that in the LN MLR; (2) depletion of spleen macrophages increased proliferation in the spleen MLR and addition of these macrophages to the LN MLR reduced proliferation; (3) macrophage depletion from the spleen MLR abolished the proliferation-enhancing effect of dbcAMP. In conclusion, cAMP enhances lymphocyte proliferation in this system, apparently as a consequence of suppressing the inhibitory influence of macrophages.     

GM-CSF诱导外周血单核细胞的分化及其对混合淋巴细胞反应的影响

作者对 Ｇ Ｍ Ｃ Ｓ Ｆ 作用下的外周血单核细胞（ Ｐ Ｍ Ｃ） 的形态特征及免疫表型进行了测定, 并观察了其对混合淋巴细胞反应（ Ｍ Ｌ Ｒ） 的影响。结果显示, Ｇ Ｍ Ｃ Ｓ Ｆ 可使外周血单核细胞分化为 Ｃ Ｄ１４ ＋ 、 Ｃ Ｄ１ａ 或低表达、 Ｃ Ｄ８０ 、 Ｃ Ｄ８６ 低表达、 Ｈ Ｌ Ａ Ｄ Ｒ＋ 、 Ｃ Ｄ１１ａ ＋ 、 Ｃ Ｄ４５ Ｒ Ａ＋ , 和非特异性酯酶呈强阳性反应的细胞, 符合巨噬细胞的形态及表型特征。在 Ｍ Ｌ Ｒ 体系中加入上述细胞, 发现其 Ｍ Ｌ Ｒ 的结果因巨噬细胞含量的不同而异, 巨噬细胞与反应细胞比例为１∶２ 时抑制 Ｍ Ｌ Ｒ（ Ｐ＜ ００５ ） , １∶４或１∶８ 时刺激 Ｍ Ｌ Ｒ（ Ｐ＜ ００５ ） ; Ｐ Ｈ Ａ 和 Ｉ Ｌ ２ 能够逆转大剂量巨噬细胞对 Ｍ Ｌ Ｒ 的抑制作用, 抗 Ｃ Ｄ３ 单抗可增强其对 Ｍ Ｌ Ｒ 的抑制作用, 消炎痛不影响巨噬细胞对 Ｍ Ｌ Ｒ 的作用。     

Inhibitory effects of tunicamycin on the mixed lymphocyte reaction and mitogen-induced lymphocyte blastogenesis

The role of asparagine(Asn)-linked saccharides on the surface of lymphocytes in cellular interactions was examined by performing studies on the effects of tunicamycin (TM), which inhibits the glycosylation of proteins N-glycosylated at asparagine residues, on the mixed lymphocyte reaction (MLR) and mitogen-induced lymphocyte blastogenesis of human lymphocytes by measuring 3H-thymidine (TdR) incorporation. Responses were expressed as percentages of that of the control (MLR without TM). The lymphocyte blastogenesis in the one- and two-way MLR were, respectively, 43.1% and 48.0% of the control at 0.1 microgram/ml of TM, and 5.5% and 7.2% at 1 microgram/ml of TM. The inhibitory effect of TM on the one-way MLR was shown using TM-pretreated stimulator cells, TM-pretreated responder cells or both. TM blocked lymphocyte blastogenesis in the secondary as well as in the primary MLR. The inhibitory effect of TM on the two-way MLR was observed when TM was added on day 0 to day 2, but not on day 4 of incubation. TM blocked mitogen-induced lymphocyte blastogenesis by phytohemagglutinin, concanavalin A or by pokeweed mitogen. As TM had no cytotoxic effect on cultured cells, these inhibitory effects of TM were thought to be due to the loss of Asn-linked saccharides from glycoprotein of the surface of lymphocytes. These findings indicated that Asn-linked saccharides of glycoprotein on the surface of lymphocytes were important in cellular interactions that are necessary for the cellular immune response.     

Action profiles of statins and calcineurin inhibitors during human mixed lymphocyte reaction

The cell-to-cell interaction through binding intercellular adhesion molecule (ICAM)-1 and CD40 on monocytes and their ligands such as lymphocyte function-associated antigen (LFA)-1 and CD40 ligand (CD40L) on T-cells plays roles in cytokine production and T-cell proliferation. Interleukin (IL)-18, which is elevated in the plasma during acute rejection after organ transplantation, induces the expression of ICAM-1 and CD40 on monocytes, the production of interferon (IFN)-gamma and IL-12 and the proliferation of T-cells during the human mixed lymphocyte reaction (MLR). In addition to the cholesterol lowering effect, statins improve patient survival and decrease rejection episodes in transplant recipients. In the present study, we investigated the difference of effect of statins and calcineurin inhibitors during MLR. 3-Hydroxy-3-methylglutaryl coenzyme-A (HMG-CoA) reductase inhibitors, fluvastatin and pravastatin and statin-derived LFA-1 inhibitors, LFA703 and LFA878, which did not inhibit HMG-CoA reductase, suppressed the production of IFN-gamma and IL-12 and the lymphocyte proliferation as well as the expression of ICAM-1 and CD40 on monocytes regardless of the presence of IL-18. However, the calcineurin inhibitors, tacrolimus and cyclosporine A (CsA), inhibited the IL-18-enhanced cytokine production and lymphocyte proliferation without any effect on the adhesion molecule expression. Thus, the action mechanism of stain is different from that of calcineurin inhibitors.     

Accessory cell function of human alveolar macrophages in antigen-induced T lymphocyte proliferation

We compared the accessory cell function of human alveolar macrophages (AM) to that of human blood monocytes (Mo) obtained by bronchoalveolar lavage and venipuncture from normal volunteers. Graded numbers of either AM or Mo were added to autologous peripheral blood T lymphocytes that were stimulated with a purified protein derivative of tuberculin (PPD). Either AM or Mo were cocultured with allogeneic T lymphocytes in mixed lymphocyte reaction (MLR) experiments. Both AM and Mo supported the PPD-induced T lymphocyte proliferation and allogeneic MLR at low ratios of AM or Mo to T lymphocytes with similar efficiency. However, AM showed marked suppressive effects at higher ratios of AM to T lymphocytes (1:1). PPD-pulsed AM, but not AM killed by physical treatments (heat, freeze-thaw, sonication), induced T lymphocyte proliferation. An indirect immunofluorescent study demonstrated that most AM express HLA-DR antigens. Furthermore, AM synthesized DR antigens with molecular weights of 33,000 and 29,000-31,000 daltons. When AM were treated with both anti-DR monoclonal antibody and complement, PPD-induced T lymphocyte proliferation and MLR were diminished. These results suggest that human AM function as accessory cells in the antigen-induced T lymphocyte proliferation and DR antigens on AM play an important role in the accessory cell function.     

Generation of cytotoxic T cells in the rat mixed lymphocyte reaction is blocked by monoclonal antibody MRC OX-8

MRC OX-8 is a mouse anti-rat monoclonal antibody which binds to thymocytes, cytotoxic T cells, suppressor cells and the majority of natural killer (NK) cells. Addition of this antibody at the beginning of a mixed lymphocyte reaction (MLR) inhibits the generation of cytotoxic T cells, assayed after 5 days. However, MRC OX-8 antibody has no effect on proliferation in the MLR, in particular of the MRC OX-8+ cells. The generation of cytotoxicity in the MLR is blocked by MRC OX-8 IgG and F(ab')2 and requires interaction of responder cells with the antibody during the first 24 hr of the MLR, indicating that it is the early stages which are most affected. MRC OX-8 has no effect on cytotoxic T cell function when added at the effector stage of the 51Cr-release assay and has no effect on NK cell-mediated killing.     

Adrenocorticotropic hormone as a potential enhancer of t-lymphocyte function in the rat mixed lymphocyte reaction

The effects of adrenocorticotropic hormone (ACTH) (10⁻⁶ to 1 U/ml) on T-lymphocyte proliferation and function were studied in the rat mixed lymphocyte reaction (MLR). ACTH produced a modest increase in lymphocyte proliferation on day 3 in lymph node (LN) cells and on day 5 in spleen cells. In addition, LN MLR cells, activated in the presence of ACTH, showed a higher proliferative response to restimulation on day 5 and on day 11 of the primary culture. Cytotoxic activity and the number of IL-2R⁺ cells were increased in ACTH-treated LN MLR cultures in experiments where control MLR levels were low. ACTH also overcame the generation of low levels of suppressor activity in spleen MLR cells. These findings indicate that ACTH could play a role in increasing the priming of T-lymphocytes and enhancing, in particular, suboptimal primary responses.     

Inhibition by anti-beta2-microglobulin of MLR by human fetal liver cells

Liver cells of fetuses without lymphoid thymus show strong, immunologically nonspecific proliferation when confronted with adult foreign lymphocytes. To elucidate the mechanism of this response, we have studied the effect of anti-β₂-microglobulin on fetal liver cells in mixed lymphocyte reaction (MLR). Anti-β₂-microglobulin inhibited MLR when antiserum was present in the culture or when the responding fetal cells had been preincubated with it. If the adult lymphocytes stimulating fetal liver cells were preincubated with anti-β₂-microglobulin, only a slight or no effect was observed. No difference was observed between liver cells from fetuses with and without lymphoid thymus. β₂-microglobulin was also demonstrated on the fetal liver cells by immunofluorescence. The results reveal that β₂-microglobulin is present on early fetal liver cells, and that MLR by fetal liver cells is inhibited by anti-β₂-microglobulin in the same way as in adults. multipotential immunocompetence of early fetal liver cells cannot be excluded on the basis of these experiments.     

Lactoferrin regulates proliferative response of human peripheral blood mononuclear cells to phytohemagglutinin and mixed lymphocyte reaction

The aim of this study was to investigate the effects of lactoferrin (LF) on the proliferative response of human peripheral blood mononuclear cells (PBMC) induced by phytohemagglutinin (PHA) and alloantigens in a two-way mixed lymphocyte reaction (MLR). The proliferative responses were measured by MTT colorimetric assay and presented as a proliferation index (PI) or as changes in PI caused by LF relative to PHA or MLR controls. We found that the effects of LF in both experimental models were differential and dependent on an individual PBMC reactivity, mitogen or alloantigen and LF concentration. Generally, lymphocytes from donors responsive to LF exhibited higher proliferation indices to PHA when compared with non-responsive individuals. Lactoferrin at low doses showed regulatory effects, whereas at higher doses the proliferation was significantly reduced. Mixed lymphocyte reaction was generally inhibited by LF. The results suggest that the differential action of LF might be due to its ability to sense the activation status of lymphocytes.     

Burn-induced immunosuppression: attenuated T cell signaling independent of IFN-gamma- and nitric oxide-mediated pathways

Burn injury results in immunosuppression; previous work implicated a combination of altered T lymphocyte subpopulations and the elaboration of macrophage-derived mediators. However, the conclusions were based on T cell stimulations in the setting of high-dose polyclonal mitogenic stimuli and a single kinetic time-point. In this study, splenocytes from burned animals were used to examine lymphocyte responses over a multi-day time course following saturating and subsaturating anti-CD3, as well as mixed lymphocyte response (MLR) stimulation. Burn injury resulted in suppressed splenocyte-proliferative responses to high-dose anti-CD3 (2 microg/ml) at all culture time-points (Days 2-5); this inhibition was eliminated by removing macrophages from the splenocyte cultures, by blocking NO production, or by using splenocytes from burned animals congenitally deficient in IFN-gamma (IFN-gamma(-/-)). The results are consistent with immunosuppression attributable to burn-induced IFN-gamma production, which in turn, drives macrophage NO synthesis (NOS). In MLR cultures, lymphocyte proliferation and IFN-gamma production were depressed at later time-points (Days 3-5). APC from burned animals showed no defects as MLR stimulators; T cells from burned animals showed defective, proliferative responses, regardless of the stimulator population. Removing macrophages, adding a NOS inhibitor, or using IFN-gamma(-/-) splenocytes did not restore the MLR response of burned splenocytes. T cells from burned IFN-gamma(-/-) animals also showed depressed proliferation with subsaturating levels of anti-CD3 (0.1 microg/ml); anti-CD-28 augmented the proliferative response. We conclude that burn-induced immunosuppression to authentic antigenic stimulation is related at least in part to defective CD3 signaling pathways and not simply to increased IFN-gamma or NO production.     

移植免疫中T细胞疫苗作用机制的初步分析

为分析Ｔ细胞疫苗同种兔疫抑制作用的机制，在体外观察了Ｔ细胞疫苗免疫小鼠细胞免疫应答能力的改变，分别在ＭＩＣ实验中，观察了免疫小鼠血清和免疫小鼠脾细胞对同种应答细胞反应能力的影响。结果表明：免疫小鼠淋巴细胞ＣｏｎＡ转化反应能力及同种抗原反应能力均显著降低，免疫小鼠血清及其脾细胞不同程度地抑制了同种应答细胞的反应能力，但以上作用并未表现出抗原特异性。     

阿司匹林对树突状细胞成熟及免疫功能的体外研究

研究阿司匹林在体外对小鼠树突状细胞(mDC)成熟及免疫功能的影响。在骨髓来源未成熟树突状细胞培养过程中,加入不同剂量(1mmol/L、2mmol/L)的阿司匹林,观察DC形态。流式细胞仪检测各组mDC表面共刺激分子CD86、CD80的表达;台盼蓝染色测细胞活力;混合淋巴细胞反应(MLR)检测mDC对T淋巴细胞刺激能力;ELISA法检测MLR上清液中细胞因子。与对照组比,阿司匹林处理的DC(aspirin-DC)表面CD80、CD86表达明显降低(P<0.01);对T淋巴细胞刺激能力减弱;MLR上清液中炎性因子(TNF-α、IL-1β)明显减少(P<0.01)。结果:在体外培养过程中给予阿司匹林干预能够抑制mDC的成熟及功能,呈剂量依赖性;阿司匹林对DC功能的抑制可能是阿司匹林抑制炎症反应,治疗冠心病的机制之一。     

Humoral inhibitors of the immune response in uremia. V. Induction of suppressor cells in vitro by uremic serum.

The mechanism of inhibition of mixed lymphocyte reaction (MLR) by serum of chronically uremic rats has been studied. The inhibitory activity of the serum has been associated with a discrete subset of very low density lipoproteins (VLDL) of Sf 100-400. The degree of the inhibitory activity of uremic serum correlates with the severity of uremia. Spleen cells from normal rats incubated for 20 hours with uremic serum or its VLDL fraction suppress the response of control syngeneic cells in the MLR. Induction of such suppressor activity does not require cell proliferation because it is not inhibited by mitomycin C. although the exact identity of the induced suppressor cells has not been established, they may be macrophages. The suppressor activity of induced spleen cells can be markedly reduced by filtration of spleen cells on glass wool or on nylon wool columns. Reconstruction experiments show that the adherent cell fraction of spleen cells exposed to uremic serum suppresses the response of the nonadherent fraction of control spleen cells. These results indicate that the immunosuppressive effects of rat uremic serum in vitro involve the induction of suppressor cells.     

IDO基因修饰的软骨细胞对T淋巴细胞增殖的影响

为检测吲哚胺2,3-双加氧酶(IDO)基因修饰的原代培养软骨细胞对T淋巴细胞的免疫抑制作用。将pEGFP-N1-IDO真核表达载体转染至C57小鼠原代培养的软骨细胞,体外检测IDO修饰的软骨细胞培养液中色氨酸水平的变化;使用CFSE标记技术,检测IDO修饰的软骨细胞体外混合淋巴细胞反应中对同种异体T淋巴细胞增殖的影响。结果:pEGFP-N1-IDO成功被导入原代培养的小鼠软骨细胞,荧光显微镜和流式细胞术均检测到EGFP的表达;体外实验表明,原代培养的软骨细胞上清可检测到一定量的色氨酸水平,而转染IDO 24 h后的软骨细胞上清中未检测到色氨酸,提示IDO转染的软骨细胞表达了具有活性IDO;使用CFSE标记技术,检测IDO修饰的软骨细胞体外混合淋巴细胞反应中对同种异体T淋巴细胞增殖的影响,发现IDO基因转染组96 h后总体增殖指数为1.122±0.017,单纯淋巴细胞阴性对照组为1.026±0.016,阳性单纯软骨细胞组为1.201±0.026,较阳性对照组,IDO基因组同种异体T淋巴细胞增殖得到明显抑制。IDO基因修饰的软骨细胞可以抑制T淋巴细胞的增殖。     

Monoclonal antibodies to B and T lymphocyte attenuator (BTLA) have no effect on in vitro B cell proliferation and act to inhibit in vitro T cell proliferation when presented in a cis, but not trans, format relative to the activating stimulus

B and T lymphocyte attenuator (BTLA) is an immunoglobulin superfamily member surface protein expressed on B and T cells. Its ligand, herpesvirus entry mediator (HVEM), is believed to act as a monomeric agonist that signals via the CRD1 of HVEM to inhibit lymphocyte activation: HVEM is also the receptor for lymphotoxin-α and LIGHT, which both bind in the CRD2 and CRD3 domains of the HVEM molecule, and for CD160 which competes with BTLA. We have shown that recombinant HVEM and a panel of different monoclonal antibodies specifically bind murine BTLA on both B and T cells and that some antibodies inhibit anti-CD3ε-induced T cell proliferation in vitro, but only when constrained appropriately with a putatively cross-linking reagent. The antibodies had no significant effect on in vitro T cell proliferation in a mixed lymphocyte reaction (MLR) assay nor on in vitro DO11.10 antigen-induced T cell proliferation. None of these antibodies, nor HVEM-Fc, had any significant effect on in vitro B cell proliferation induced by anti-immunoglobulin M antibodies (±anti-CD40) or lipopolysaccharide. We further elucidated the requirements for inhibition of in vitro T cell proliferation using a beads-based system to demonstrate that the antibodies that inhibited T cell proliferation in vitro were required to be presented to the T cell in a cis, and not trans, format relative to the anti-CD3ε stimulus. We also found that antibodies that inhibited T cell proliferation in vitro had no significant effect on the antibody captured interleukin-2 associated with the in vivo activation of DO11.10 T cells transferred to syngeneic recipient BALB/c mice. These data suggest that there may be specific structural requirements for the BTLA molecule to exert its effect on lymphocyte activation and proliferation.     

Expanded adipose-derived stem cells suppress mixed lymphocyte reaction by secretion of prostaglandin E2

Multipotent mesenchymal stem cells (MSCs) in adult tissue are known to be less immunogenic and immunosuppressive. Previous study showed that primary cultures of human adipose-derived stem cells (ADSCs) shared their immunomodulatory properties with other MSCs. However, whether passaged human ADSCs can retain their immunomodulatory effect after in vitro expansion remains unknown. In addition, the mechanism of ADSC-mediated immunomodulatory effect remains to be elucidated. This study aimed to investigate these issues by using passaged human ADSCs as an in vitro study model. Flow cytometry showed that passaged ADSCs expressed human leukocyte antigen (HLA) class I but not class II molecules, which could be induced to express to a high level with interferon-gamma (IFN-gamma) treatment. The study found that passaged ADSCs could not elicit lymphocyte proliferation after co-culturing with them, even after IFN-gamma treatment. In addition, either IFN-gamma-treated or non-treated ADSCs could inhibit phytohemagglutinin (PHA)-stimulated lymphocyte proliferation. Moreover, passaged ADSCs could serve as the third-party cells to inhibited two-way mixed lymphocyte reaction (MLR). Further study using a transwell system also showed that this type of immunosuppressive effect was not cell-cell contact dependent. In defining possible soluble factors, we found that passaged ADSCs significantly increased their secretion of prostaglandin E2 (PGE2), but not transforming growth factor-beta (TGF-beta) and hepatocyte growth factor (HGF), when they were co-cultured with MLR. Furthermore, the result demonstrated that only PGE2 production inhibitor indomethacine, but not TGF-beta- and HGF-neutralizing antibodies, could significantly counteract ADSC-mediated suppression on allogeneic lymphocyte proliferation. These results indicated that in vitro expanded ADSCs retain low immunogenicity and immunosuppressive effect, and PGE2 might be the major soluble factor involved in the in vitro inhibition of allogeneic lymphocyte reaction.     

Humoral inhibitors of the immune response in uremia. I. Effect of serum and of the supernatant of spleen cultures from uremic rats on the mixed lymphocyte reaction.

The effects of the serum of rats with experimentally induced chronic renal failure and of the supernatant of cultured spleen cells obtained from these animals were tested in the mixed lymphocyte reaction (MLR). Serum of uremic rats with blood urea nitrogen levels greater than 35 mg. per 100 ml. and supernatant of spleen cultures from animals with blood urea nitrogen levels greater than 60 mg. per 100 ml. were found to be inhibitory to the MLR. The inhibitory activity of uremic serum was directly correlated with blood urea nitrogen levels. The inhibitory activities both of uremic serum and of the supernatant of cultured uremic spleen cells were further studied and compared. Dialysis in vitro does not remove the suppressing effect of serum or of supernatant on the MLR. Irreversible inhibition of the MLR is observed after 48 hours' exposure to the serum or to the supernatant; this inhibition occurs whether the MLR culture is exposed during the first 48 hours or last 48 hours of incubation. In some uremic rats the inhibitory activity of the serum disappears or is weakened after splenectomy, despite a continued rise in blood urea nitrogen levels.     

Complement inhibits immune responses: C3 preparations inhibit the generation of human cytotoxic T lymphocytes

C3 fragments have been shown to inhibit mitogen- and antigen-induced human lymphocyte blastogenesis. In this study, C3 preparations and a small fragment of C3 (contained in a preparation called Fraction 2) were examined for their capacity to regulate cytotoxic T lymphocytes (CTL) and proliferative mixed lymphocyte responses (MLR). We found that both preparations inhibited the generation of allogeneic human CTL as well as MLR in a dose-related manner. In contrast, Fraction 1, which contained native C3 and C3b, did not inhibit the generation of CTL nor did it inhibit the MLR. The kinetics of inhibition of proliferation were divergent from the kinetics of inhibition of CTL generation; the active preparations inhibited proliferation significantly when added as late as day 5 of a 6-day culture, whereas no inhibition of CTL generation was seen when these preparations were added after day 3 of culture. Cultures in which C3 preparations caused complete inhibition of CTL generation had normal levels of the nonspecific, anomalous killer cell activity, as assayed on K 562 target cells. Furthermore, C3 preparations and Fraction 2 had no effect on the lytic function of differentiated CTL, on “spontaneous” natural killer cell activity or on interferon-induced augmentation of natural killer cell activity. These findings indicate that C3 fragments may play a negative role in the regulation of CTL responses.     

Effects of a single donor-specific blood transfusion on the survival of rat cardiac allografts

It is now well recognized that pre-transplant donor-specific blood transfusion (DST) has a beneficial effect on the survival of allografts. To determine the optimal interval between DST and transplantation, and to analyze the mechanisms of this effect, the survival of cardiac allografts to rats which received a single DST was examined. The cardiac allograft survival was found to be prolonged when the DST was performed 1 to 6 weeks before grafting. In addition, recipient rat sera collected 1 to 6 weeks after a single DST showed significant inhibition of a mixed lymphocyte reaction (MLR). This MLR inhibition correlated with prolongation of survival of histoincompatible rat cardiac allografts. It thus appears that a single DST given from 1 to 6 weeks before transplantation has a beneficial effect on allograft survival and that MLR inhibition may be essential for inducing the effect of transfusion on organ transplantation.     

Thyroglobulin-induced T-cell in vitro proliferation in Hashimoto's thyroiditis: identification of the responsive subset and effect of monoclonal antibodies directed to Ia antigens

Recently it was reported that the peripheral blood and thyroid gland of patients with Hashimoto's thyroiditis contain activated (Ia+ and/or MLR4+) T cells and high levels of 5/9+ ("helper") T lymphocytes. In normal individuals the 5/9 monoclonal antibody recognizes a T-cell fraction that includes all T lymphocytes with inducer activities. Here, circulating 5/9+ and 5/9- T lymphocytes were isolated from patients with Hashimoto's disease, and the proliferative response induced by human thyroglobulin was investigated. The results show that the total thyroglobulin-induced lymphocyte DNA synthesis is confined to the 5/9+ T-cell fraction. Further subfractionation of 5/9+ into MLR4+ and MLR4- cells clearly indicates that no substantial differences exist in their proliferative capacities. Whether 5/9, MLR4, and Ia antigens, all expressed on the thyroglobulin-responsive T-cell subset, are involved in thyroglobulin-induced cell proliferation, was also analyzed. Although both 5/9 and MLR4 monoclonal antibodies had no effect, complete inhibition of antigen-induced blastogenesis was observed upon addition of monoclonal antibodies (D1/12 and BT2/9) directed to common determinants of Ia antigens. This inhibitory effect was also observed when T or non-T fractions were separately incubated with the monoclonal antibodies before culture. These results indicate that in humans, as in animals, the major histocompatibility complex may play a role in autoimmune thyroiditis. The data show that (a) the thyroglobulin-induced proliferative response is confined to a subset (5/9+) of T lymphocytes and (b) Ia antigens are involved in thyroglobulin-induced lymphocyte DNA synthesis in Hashimoto's disease.