人单核细胞共刺激分子在异种免疫反应中的表达及其作用机制

目的 揭示人单核细胞共刺激分子在异种免疫反应中的表达及其作用机制.方法 从猪的主动脉分离血管内皮细胞(PEC)并培养扩增;从人单个核细胞(PBMC)中纯化CD4+T淋巴细胞和单核细胞.建立PEC和人PBMC混合培养体系,培养后收集细胞,然后加入荧光标记的单克隆抗体,通过流式细胞术检测CD14+单核细胞表面共刺激分子表达情况.为了检测淋巴细胞增殖反应以及阻断共刺激分子对PEC免疫反应的作用,在PEC和人PBMC混合培养体系中分别加入抗CD154、CD80和CD86单克隆抗体.在培养的最后24 h加入同位素,于培养结束后收集细胞并经同位素计数仪进行检测.纯化的单核细胞经PEC刺激后与CD4+T淋巴细胞共培养来研究这些单核细胞诱导CD4+T淋巴细胞的增殖以及阻断共刺激分子的作用.结果 PEC和人PBMC混合培养后可检测到PBMC对异种PEC的高度免疫增殖反应;流式细胞术检测到PBMC中的CD14+单核细胞表面无CD40和CD80的表达,但表达CD86,经PEC刺激后,CD14+单核细胞膜表面显著上调CD40和CD80蛋白分子的表达,CD86表达上调.与未经刺激的单核细胞相比较,经PEC刺激后的单核细胞和CD4+T淋巴细胞共培养后可诱导CD4+T淋巴细胞明显增殖,抗人CD154、CD80、CD86单克隆抗体可以阻断CD4+T淋巴细胞对PEC的增殖反应.结论 人CD14+单核细胞在异种免疫反应过程的间接抗原提呈和共刺激信号传导中发挥重要作用,通过上调其共刺激分子的表达与CD4+T淋巴细胞共刺激分子CD154和CD28相互作用形成第二信号,并诱导CD4+T淋巴细胞对PEC的增殖反应;阻断共刺激分子可抑制异种细胞免疫反应.     

新一代多克隆抗体抑制异种细胞免疫反应的研究

目的 研究新一代兔抗人免疫细胞多克隆抗体(newRALG)对异种细胞免疫反应的抑制作用.方法 应用活化的淋巴细胞和单核细胞致敏新西兰兔后获得newRALG.将PKH-26标记的猪血管内皮细胞(PEC)和正常人单个核细胞(PBMC)建立混合培养体系,培养液中分别加入newRALG、正常兔IgG、Thymoglobulin和Scavenger受体(SR)阻断剂Poly G,培养后收集PBMC,然后分别加入异硫氰基荧光素(FITC)标记的鼠抗人CD14、CD40、CD80、CD86和HLA-DR单克隆抗体.通过流式细胞术检测单核细胞对PEC膜的吞噬作用;通过淋巴细胞与PEC的混合培养体系,加入newRALG,以观察淋巴细胞的增殖反应和newRALG对增殖反应的阻断作用.结果 PBMC与PKH-26标记的PEC共培养后,PBMC中的CD86+单核细胞表达PKH-26,进一步研究发现PKH-26阳性的CD86+单核细胞不但表达CD14、CD86和HLA-DR,同时上调共刺激分子CD40和CD80的表达水平.正常兔IgG(作为阴性对照)对单核细胞吞噬PEC膜无阻断作用,Thymoglobulin具有较低的阻断效果,newRALG与Poly G相似,均具有较高的阻断作用.正常未经刺激的人淋巴细胞不具备增殖能力,经灭活的PEC刺激后,人淋巴细胞具有高水平的免疫增殖反应.正常兔IgG不能阻断PEC对人淋巴细胞的免疫增殖反应;Thymoglobulin的抑制作用随浓度的降低而增强;高浓度的newRALG不能抑制人淋巴细胞的免疫增殖反应,但低浓度的newRALG则显示出强大地抑制人淋巴细胞免疫增殖反应的作用.结论 单核细胞在异种免疫反应过程中发挥重要作用;newRALG可有效抑制单核细胞的吞噬功能和淋巴细胞的免疫增殖反应,从而有效的抑制异种移植排斥反应.     

核转录因子-κB通路在CD40-CD154诱导血管内皮细胞活化中的作用

目的 探讨血管内皮细胞(VEC)内核转录因子-κB(NF-κB)通路在CD40-CD154相互作用诱导VEC活化中的作用.方法 应用重组人肿瘤坏死因子α(rhTNF-α)和重组人γ干扰素(rhIFN-γ)刺激人主动脉VEC,通过流式细胞仪(FACS)检测CD40和粘附因子表达水平;通过表达CD154的细胞株D1.1和VEC共同培养后,观察VEC粘附因子的表达水平;通过抗CD154单克隆抗体阻断CD40-CD154间的反应;应用NF-κB阻断剂BAY11-7082观察阻断NF-κB通路后对CD40-CD154诱导VEC活化的影响.结果 未活化的VEC膜表达低水平的CD40、CD54和CD106,但不表达CD62E,经白细胞介素刺激后可上调这些蛋白分子的表达水平.D1.1细胞株和VEC共同培养后可诱导VEC活化并上调CD62E、CD54和CD106的表达水平;抗CD154抗体可阻断CD40-CD154间的反应所诱导的VEC活化;NF-κB阻断剂可阻断rhTNF-α诱导的VEC活化以及CD40-CD154间的反应所诱导的VEC活化.结论 VEC中CD40和T淋巴细胞中CD154之间的相互作用在诱导VEC活化过程中起重要作用;CD40-CD154相互作用并通过NF-κB通路诱导VEC活化;NF-κB通路阻断剂可抑制VEC活化.     

可溶性CD40分子与EGFP融合基因的构建及在树突状细胞中的表达及功能鉴定

CD40与T细胞表面对应的配体CD154（CD40L）相互作用后，可以进一步刺激抗原提呈细胞，上调其CD80／CD86的表达，促进T细胞的活化。可溶性CD40蛋白（soluble CD40，sCD40）由CD40信号肽和胞外功能区构成，能够与膜表面CD40竞争性结合CD40L的功能区域。     

使用他克莫司的肝移植受者T淋巴细胞亚群及CD28家族共刺激分子的分析

目的 观察使用他克莫司(Tac)的肝移植受者外周血中T淋巴细胞亚群及CD28家族共刺激分子表达的变化及其意义.方法 以接受同种异体肝移植的受者20例为Tac组,术后采用Tac、霉酚酸酯(或硫唑嘌呤)和糖皮质激素预防排斥反应;以等待肝移植的终末期肝脏疾病患者20例为肝病对照组;以健康志愿者20名为正常对照组.测定正常对照组志愿者和肝病对照组患者的外周血中T淋巴细胞亚群及其CD28、可诱导共刺激分子(ICOS)、CD152和程序性死亡因子1(PD-1)的表达;分别测定Tac组使用Tac后2周、1个月、2个月和3个月时外周血中T淋巴细胞亚群及其CD28、ICOS、CD152和PD-1的表达.结果 肝病对照组、正常对照组和Tac组各时点间的CD3+ T淋巴细胞的差异无统计学意义(P>0.05).随着使用Tac时间的延长,Tac组CD4+ T淋巴细胞持续下降,至使用Tac 3个月时,明显低于正常对照组(P<0.05);CD8+ T淋巴细胞逐渐增加至正常水平.Tac组T淋巴细胞亚群表面ICOS分子表达持续低于正常对照组(P<0.05),CD4+ T淋巴细胞表面CD28分子表达恢复至正常水平(P>0.05),而CD8+ T淋巴细胞表面CD28分子持续低表达(P<0.05).同时,Tac组T淋巴细胞亚群表面CD152分子和PD-1分子的表达均持续高于正常对照组(P<0.05).结论 肝移植患者接受以Tac为主的免疫抑制治疗,其T淋巴细胞亚群平衡紊乱得到纠正,T淋巴细胞亚群表面CD28分子和ICOS分子的表达下调,CD152分子和PD-1分子的表达上调,通过共刺激信号的调节来达到抑制排斥反应的目的.     

供体脾细胞联合抗CD154输注诱导肺移植免疫耐受

目的 探讨供体特异性输注(DST)联合共刺激阻断(DST/抗CD154)诱导肺移植后闭塞性细支气管炎(OB)免疫耐受机制.方法 建立小鼠DST/抗CD154方案诱导的免疫耐受模型术后15、25、100 d取出移植气道检测形态学改变,利用CSME-80鼠cDNA芯片检测15d耐受组和同种异体移植组移植物内基因表达差异,分析差异有统计学意义的基因表达与排斥和耐受之间的关系.选择部分差异表达基因行实时荧光定量逆转录-聚合酶链反应( RT-qPCR),鉴定验证基因芯片的可靠性.结果 免疫耐受组OB前期存在大量显著表达差异基因,Rapl信号通路、CD40/CD40L相关及其他T细胞表面分子相关基因、细胞周期、细胞生长等促进纤维增殖的部分基因显著下调表达.部分促炎因子同同种异体移植组一样上调表达.结论 Rap1信号通路、CD40/CD40L相关及其他T细胞表面分子相关基因、细胞周期、细胞生长等促进纤维增殖下调表达的部分基因等可能与肺移植慢性排斥反应耐受相关.     

阻断OX40和CD40协同共刺激信号延长小鼠胰岛移植物存活时间

目的 研究阻断OX40/OX40L和CD40/CD154L协同共刺激通路对小鼠胰岛移植物存活的影响及其机制.方法 以DBA/2小鼠为供者,C57BL/6小鼠为受者,制作胰岛移植模型.受鼠分为4组.(1)对照组,注射IgG; (2)抗OX40组,注射抗OX40L单克隆抗体;(3)抗CD154组,注射抗CD154单克隆抗体;(4)联合治疗组,注射抗OX40L单克隆抗体和抗CD1 54单克隆抗体.记录各组胰岛移植物平均存活时间(MST).将CD154敲除小鼠处死,取其脾脏T淋巴细胞,体外检测活化T淋巴细胞表面OX40的表达;在活化T淋巴细胞中加入不同浓度的抗OX40L单克隆抗体,体外检测T淋巴细胞增殖情况.结果 对照组胰岛移植物MST为19 d,抗CD154组胰岛移植物MST为48 d(P＜0.05);抗OX40组胰岛移植物MST为22 d,与前两组相比较,差异无统计学意义(P＞0.05);联合治疗组胰岛移植物MST＞ 150 d,高于另外3组(P＜0.05).66％的胞表达OX40,较初始T淋巴细胞的表达率高(2％,P＜0.05);加入抗OX40L单克隆抗体后,T淋巴细胞增殖受抑制且呈剂量依赖性.结论 阻断OX40/OX40L和CD40/CD154L双通路可诱导小鼠胰岛移植物长期存活, 其发挥作用的关键机制是抑制了T淋巴细胞的增殖.     

大鼠胰腺癌微环境诱导未成熟树突状细胞功能的变化

目的 观察胰腺癌微环境对树突状细胞(DC)成熟的影响及功能变化并探讨胰腺肿瘤细胞免疫逃逸的机制.方法 培养树突状细胞,加入粒细胞巨噬细胞集落刺激因子(rmGM-CSF)40μg/L、白细胞介素(IL)-4 40μg/L,培养到第6天时得到大量的未成熟树突状细胞(imDC),加入大鼠胰腺癌细胞(AR42J cell)培养上清液诱导,流式细胞术检测DC的表面分子CD86、CD80的表达(n=6),观察能否延缓或阻断imDC的成熟及其在脂多糖(LPS)刺激后这种作用能否被逆转.并观察AR42J细胞培养上清诱导的Dc对同种异体混合淋巴细胞增殖.结果 加入胰腺癌癌细胞上清液培养的DC,与正常成熟的DC比较,CD80+CD86+阳性率由(70.88±3.60)%降至(7.15±0.71)%,LPS刺激后DC细胞的表面分子CD80+CD86+表达仍然较低(7.43±1.05)%,表明胰腺癌细胞培养上清液对DC的成熟有阻断作用.AR42J细胞上清诱导培养的imDC组刺激同种异体混合淋巴细胞增殖的强度显著低于正常培养的imDC组刺激同种异体混合淋巴细胞增殖的强度.结论 体外大鼠胰腺癌细胞培养上清液可以诱导DC处于不成熟状态,且这种不成熟状态不容易被逆转.     

导入细胞毒性T淋巴细胞相关抗原4融合蛋白基因抑制大鼠移植肝组织中穿孔素mRNA的表达

研究表明,CD28-B7共刺激途径在T淋巴细胞活化中起主要作用,应用细胞毒性T淋巴细胞相关抗原4融合蛋白(CTLA4Ig)分子阻断CD28-B7信号传导通路,导致T淋巴细胞对抗原的特异性免疫无反应性,从而抑制同种异体器官移植排斥反应.     

小鼠补体调节蛋白对CD4+T淋巴细胞的调控作用及其机制

目的 研究小鼠补体调节蛋白Crry对CD4+T淋巴细胞的调控作用及诱导同种移植免疫低反应性的机制.方法 分离C57BL/6小鼠脾淋巴细胞,用免疫磁珠法分选出CD4+T淋巴细胞后,将CD4+T淋巴细胞分为A、B、C、D、E和F组,分别用抗小鼠CD3、CD28、Crry、CD3/CD28、CD3/Crry和CD3/CD28/Crry抗体共刺激通路与CD4+T淋巴细胞进行反应,采用噻唑蓝(MTT)法检测各组CD4+T淋巴细胞的增殖情况,并采用酶联免疫吸附试验检测CD4+T淋巴细胞培养上清中白细胞介素2(IL-2)、γ干扰素(γ-IFN)、IL-4和IL-10的水平;另外,以BALB/c小鼠和C57BL/6小鼠的脾细胞分别作为刺激细胞和反应细胞,建立同种混合淋巴细胞反应(MLR)体系并加入抗小鼠Crry抗体,通过岍法观察Crry对MLR的影响.结果 D、E、F组的CD4+T淋巴细胞均出现明显增殖,增殖活性显著高于A、B、C组(P<0.05),其中F组显著高于D组和E组(P<0.05),D组和E组间增殖活性的差异无统计学意义.D组CD4+T淋巴细胞经抗CD3/CD28抗体共刺激后,培养上清中γ-IFN和IL-2的水平显著升高,与A、B、C和E组比较,差异均有统计学意义(P<0.05),但与F组的差异无统计学意义;E组CD4+T淋巴细胞经抗CD3/Crry抗体共刺激后,IL-4的水平显著升高,与A、B、C、D组比较,差异均有统计学意义(P<0.05),但显著低于F组(P<0.05);各组间IL-10水平的差异无统计学意义.Crry可以明显抑制MLR中的细胞增殖(P<0.05).结论 补体调节蛋白Crry能刺激CD4+T淋巴细胞的增殖,并使其IL-4的表达升高及抑制IL-2和γ-IFN的表达,从而诱导同种移植免疫低反应性.     

A nuclear factor-kappaB inhibitor BAY11-7082 inhibits interactions between human endothelial cells, T cells, and monocytes

Costimulatory molecules play critical roles during cell-mediated immune responses. We undertook this study to determine whether CD154-CD40 interactions induced human endothelial cell (EC) activation via the nuclear factor (NF)-κB pathway, and whether the upregulation of monocyte-derived CD40 and CD80 is NF-κB pathway dependent. A CD154-expressing D1.1 cell-EC coculture with or without the NF-κB inhibitor BAY11-7082 was established to examine EC activation as indicated by CD62E expression. Peripheral blood mononuclear cell (PBMC)-EC cocultures were performed in the presence or absence of BAY11-7082; the expression of CD40 and CD80 on monocytes was analyzed by FACS. Allogeneic mixed lymphocyte-EC reaction (MLER) was performed to determine the inhibitory effects of BAY11-7082 to prevent lymphocyte proliferation. FACS demonstrated upregulation of EC-derived CD62E expression induced by CD154 expressing D1.1 cells. BAY11-7082 pretreated EC failed to upregulate CD62E after interaction with D1.1 cells. Monocytes upregulated CD40 and CD80 expression during PBMC-HEC interaction, and BAY11-7082 suppressed monocyte-derived CD40 and CD80 expression in a dose-dependent manner. The monocyte-derived CD86 expression was downregulated by NF-κB inhibitor. BAY11-7082 demonstrated inhibition of lymphocyte proliferation of allogeneic MLER. This study demonstrated that the NF-κB inhibitor BAY11-7082 prevented CD154-CD40 interaction-induced EC activation, suggesting that the activation of EC by T-cell-derived CD154 is via NF-κB pathway. The NF-κB inhibitor suppressed upregulation of monocytederived CD40 and CD80. Additionally, BAY11-7082 suppressed lymphocyte proliferation in response to allogeneic EC. These data indicated that NF-κB plays an important role in regulating costimulatory molecules in allogeneic immune responses, and strengthens the rationale for the use of NF-κB-directed therapy in allotransplantation.     

Monocyte-derived CD40 expression is regulated by interferon-γ/interferon-γ receptor-1 pathway when acting as a bridge during their interaction with T cells and allogeneic endothelial cells

Previous studies have showed the lack of CD40 expression on monocytes during monocyte and endothelial cell (EC) interaction in the absence of T cells indicating that the interaction between T cells, monocytes, and ECs is required for monocyte-derived CD40 expression. We investigated the role of monocytes acting as a bridge between ECs and T cells and the possible mechanisms for monocyte-derived CD40 up-regulation in allogeneic immune responses. A coculture system with tanswell was established between purified monocytes, T cells, and ECs, and the cells were analyzed by flow cytometry to detect monocyte-derived CD40 expression. Purified monocytes stimulated by ECs did not show up-regulation of CD40 expression. Ec-stimulated monocytes up-regulated interferon (IFN)-γ receptor-1 expression. Monocytes, stimulated by ECs, up-regulated CD40 expression in the presence of T cells. However, when T cells were separated from monocyte-EC interaction, these monocytes did not show CD40 up-regulation. Furthermore, IFN-γ receptor-1 blockade but not IFN-γ receptor-2 blockade inhibited monocyte-derived CD40 expression during monocyte-EC-T cell interaction. Neutralizing antibody directed to IFN-γ inhibited up-regulation of monocyte-derived CD40. We showed here that the interaction between T cells and EC-stimulated monocytes and up-regulation of monocyte-derived CD40 expression are contact-dependent, suggesting that monocytes act as bridge between ECs and T cells. The IFN-γ receptor-1 blockade inhibited the monocyte-derived CD40 up-regulation. These data suggest that the Th1 lymphocytes provide help for monocytes via IFN-γ and IFN-γ receptor-1 pathway following their interaction.     

CD4+ T-cell and monocyte interdependence during discordant xenoimmune responses

This study was designed to examine our hypothesis that human monocytes provide missing constimulatory signals to host CD4⁺ cells during interactions with porcine endothelial cells (PECs). PECs were isolated from the aorta. Human CD4⁺ T cells and monocytes were purified from peripheral blood mononuclear cells (PBMCs). A xenogeneic mixed lymphocyte-PEC reaction (xMLER) was performed to determine the proliferation of PBMCs or CD4⁺ cells in response to PEC. Monocyte-PEC cocultures with or without CD4⁺ cells were followed by analysis using fluorescence-activated cell scanning (FACS). We evaluated the CD4⁺ cells proliferation induced by PEC-conditioned monocytes with or without costimulation blockade. xMLER demonstrated strong lymphocyte proliferation in response to PECs. However, purified CD4⁺ cells showed reduced proliferative responses to PECs when compared with PBMCs. FACS analysis found that CD14⁺ monocytes up-regulated CD40 and CD80 expressions in the presence of CD4⁺ cells. PEC-activated but not resting monocytes induced CD4⁺-cell proliferation, which was inhibited by anti-CD154, anti-CD80, or anti-CD86 antibodies. In summary, human monocytes exposed to PECs are conditioned to up-regulate costimulatory molecules upon exposure to T cells. PEC-conditioned monocytes induced T-cell proliferation by indirect presentation. Costimulation blockade inhibited T-cell proliferation induced by PEC-conditioned monocytes. Our findings suggested that monocytes play an important role in indirect xenoantigen presentation, providing costimulation to T cells. This interaction can occur distant from the initial site of xenoantigen, but monocytes remaide void of costimulatory signals until their interaction with T cells.     

Costimulatory molecules are active in the human xenoreactive T-cell response but not in natural killer-mediated cytotoxicity

BACKGROUND: T-cell costimulatory blocking agents inhibit allospecific T-cell responses in vitro and prevent allograft rejection in vivo. Costimulatory requirements for discordant xenospecific cellular responses remain undefined. We have evaluated costimulatory molecule expression by porcine endothelial cells (PEC) after interaction with human cells and tested agents known to inhibit allospecific responses for their ability to inhibit xenospecific responses in vitro. METHODS: Human-specific agents were screened for their ability to bind porcine costimulatory molecules by FACS. Up-regulation of B7 molecules on PEC was evaluated by FACS after exposure to human cells or supernatants. The effect of human and/or porcine costimulatory blockade was tested in xeno-mixed lymphocyte reactions (XMLRs) and in natural killer (NK) cell cytotoxicity assays. RESULTS: B7 expression was induced on PEC after exposure to human T and NK cells or T cell-conditioned medium. The human XMLR was attenuated by human CTLA4-Ig and anti-human CD154 (hu5C8), and the combination was synergistic. Anti-human CD80 and CD86 antibodies alone had minor effects in the XMLR, but in combination with hu5C8 were as effective as human CTLA4-Ig plus hu5C8. Anti-hCD80 and hCD86 antibodies that did not cross-react with porcine CD80 or CD86 were as effective in blocking the MLR as those that did cross-react, indicating that the predominant costimulation in vitro was derived from the responding cells. None of the agents affected the xeno-NK response. CONCLUSIONS: We conclude that the costimulation-modulating agents block human anti-porcine T-cell responses in vitro predominantly through interruption of costimulation derived from responding cells. They have no effect on NK cell-mediated cytotoxicity.     

Costimulation and Autoimmune Diabetes in BB Rats

Costimulatory signals regulate T-cell activation. To investigate the role of costimulation in autoimmunity and transplantation, we studied the BB rat model of type 1 diabetes. Diabetes-prone BB (BBDP) rats spontaneously develop disease when 55–120 days of age. We observed that two anti-CD28 monoclonal antibodies (mAb) with different functional activities completely prevented diabetes in BBDP rats. Anti-CD154 mAb delayed diabetes, whereas treatment with CTLA4-Ig or anti-CD80 mAb accelerated disease. Anti-CD86 or anti-CD134L mAbs had no effect. Diabetes resistant BB (BBDR) rats are disease-free, but >95% of them develop diabetes after treatment with polyinosinic-polycytidylic acid and an mAb that depletes Treg cells. In the induced BBDR model, anti-CD154 mAb delayed onset of diabetes, whereas CTLA4-Ig, anti-CD134L or either of the anti-CD28 mAbs had little or no effect. In contrast, blockade of the CD134-CD134L pathway was highly effective for preventing autoimmune recurrence against syngeneic islet grafts in diabetic BBDR hosts. Blockade of the CD40-CD154 pathway was also effective, but less so. These data suggest that the effectiveness of costimulation blockade in the treatment of type 1 diabetes is dependent on both the costimulatory pathway targeted and the mechanism of induction, stage, intensity and duration of the pathogenic process.     

Analysis of Intragraft Gene and Protein Expression of the Costimulatory Molecules, CD80, CD86 and CD154, in Orthotopic Liver Transplant Recipients

CD40-CD154 and/or CD28-CD80/86 costimulatory blockade induces long-term allograft survival in numerous animal models. Studies examining the expression of costimulatory molecules during acute cellular rejection (ACR) have been limited to renal and cardiac allografts. The aim of this study was to describe the relationship between intragraft costimulatory molecule expression in OLT recipients and ACR. Forty-five liver biopsies were obtained at reperfusion and day 7. Gene and protein expression of CD80, CD86 and CD154 were analyzed by RT-PCR and immunohistochemistry. CD154 protein expression was present in 13 of 18 patients with a RAI score of 4, but in only two of 14 patients with a RAI score of <4. There was a strong association between the RAI score and the presence of CD80 and CD154 immunoreactivity. CD86 protein expression did not correlate with the severity of ACR. In reperfusion biopsies CD154, but not CD80 or CD86, protein expression correlated with the total ischaemic time. There was no association between expression of costimulatory molecule genes and ACR. In conclusion, we have demonstrated an association between CD154 and CD80 protein expression and ACR in orthotopic liver allografts.     

The interleukin-2 receptor α chain (CD25) plays an important role in regulating monocyte-derived CD40 expression during anti-porcine cellular responses

Long-term xenograft survival is limited by delayed xenograft rejection, and monocytes are thought to play an important role in this process. Although typically considered a T cell surface marker, interleukin 2 the receptor chain CD25 is also functional on monocytes. We hypothesized that CD25 expression on monocytes functions to augment monocyte activation in xeno-specific cellular responses. Xenogeneic mixed lymphocyte-endothelial cell reactions were used to study the role of CD25 in facilitating xenogeneic cell-mediated immune responses an in vitro. We also tested the effect of the anti-CD25 antibody daclizumab on monocyte-mediated T cell activation during xeno-specific cellular responses. Co-culture with porcine endothelial cells (PEC) elicited a pronounced proliferative response by human peripheral blood mononuclear cells (PBMC) that was accompanied by upregulation of CD25 and CD40 on CD14⁺ monocytes. CD4⁺ cells proliferated in response to PEC-conditioned monocytes, while blockade of CD25 with daclizumab reduced CD4⁺ cell proliferation in the presence of PEC-conditioned monocytes. In addition, daclizumab inhibited proliferation of PBMC in responses to PEC. Analysis of monocytes from PBMC-PEC cocultures by flow cytometry indicated that daclizumab inhibited CD40 upregulation on PEC-activated monocytes. These data demonstrate that CD25 blockade prevents xenogeneic cellular responses by directly blocking CD25 expression on both activated T cells and monocytes. CD25 blockade on T cells or monocytes may indirectly affect upregulation of CD40 on xenoreactive monocytes. Our data strengthen the rationale for incorporating CD25 directed therapy in discordant xenotransplantation.