表达CD19^＋CD56^-的多发性骨髓瘤浆细胞免疫表型的鉴别

目的：研究多发性骨髓瘤患者浆细胞和对照组正常浆细胞免疫表型,有效地识别表达CD19^＋CD56^-的多发性骨髓瘤恶性浆细胞。方法：采用四色流式细胞仪（BD,FACSCalibur）检测44例MM患者浆细胞以及25例健康骨髓捐献者正常浆细胞膜上的抗原表达。采用CellQuest软件分析结果。细胞膜表面抗原表达率大于20％定义为表达阳性。阳性率为表达阳性的患者及健康对照者所占百分比。结果：44例MM患者浆细胞免疫表型表达频率为：CD^138＋;97．72％（43／44）、CD38^＋：100％（44／44）、CD56^＋：63．64％（28／44）、CD19：84．09％（37／44）、CD200^＋：77．27％（34／44）、CD28^＋：38．64％（17／44）;25例对照组正常浆细胞免疫表型表达频率为：CD138^＋：100％（25／25）、CD38^＋：100％（25／25）、CD56^＋-：100％（25／25）、CD19^＋：96％（24／25）、CD200^＋：0％（0／25）、CD28^＋：0％（0／25）。在44例MM患者中,9％（4／44）患者表达CD19^＋CD56^-,利用CD200检测4例表达CD19^＋CD56^-的患者,有3例患者伴有CD200阳性表达,可与正常浆细胞鉴别。结论：CD200有利于鉴别表达CD19＋CD56-MM恶性浆细胞与正常浆细胞。     

CD56和CD117浆细胞免疫表型与多发性骨髓瘤患者染色体核型和预后的研究

目的:探讨CD56和CD117浆细胞免疫表型与多发性骨髓瘤患者染色体核型和预后的关系。方法:选取2011年1月至2017年3月我院收治的66例多发性骨髓瘤患者,均采用以硼替佐米为基础的VTD化疗方法。采集患者新鲜骨髓液,采用流式细胞术(FCM)和荧光原位杂交技术(FISH)检测浆细胞免疫表型和细胞染色体核型。分析浆细胞免疫表型与患者染色体核型和预后的关系。结果:66例患者浆细胞CD19+、CD20+、CD45+、CD56+、CD117+表达频率分别为7.6%(5/66)、18.2%(12/66)、45.5%(30/66)、66.7%(44/66)和40.9%(27/66)。20例行FISH检测的患者,18例(90.0%)核型异常,其中12例(66.7%)IgH重排,9例(50.0%)1q21+扩增,8例(44.4%)del(13q14.3)缺失,10例(55.6%)del(13q14)缺失,3例(16.7%)del(17p)缺失。CD56+患者1q21+扩增和del(13q14.3)发生率显著低于CD56-患者(27.3%vs 85.7%,18.2%vs 85.7%,P0.05)。CD117+患者1q21+扩增和del(17p)发生率显著低于CD117-患者(12.5%vs 80.0%,12.5%vs 20.0%,P0.05)。CD56+患者的PFS和OS明显延长[23.4(2.0-91.4)月vs 19.8(3.0-85.1)月,34.5(8.9-96.5)月vs 30.1(6.7-84.3)月,P0.05]。CD117+患者PFS和OS明显延长[22.9(1.0-94.3)月vs 20.3(2.0-84.3)月,33.9(7.4-93.5)月vs 31.4(6.7-89.7)月,P0.05]。Kaplan-Meier分析CD56和CD117阳性与阴性患者的PFS曲线和OS曲线存在显著性差异(P0.05)。结论:CD56+和CD117+患者的预后明显优于CD56-和CD117-患者,CD56-和CD117-患者染色体异常核型的发生率明显增加。     

160 例多发性骨髓瘤患者免疫表型特征及其临床意义

目的:研究多发性骨髓瘤(MM)细胞免疫表型特征及其临床意义。方法:应用直接免疫荧光抗体染色方法,采用四色流式细胞仪分析160例多发性骨髓瘤患者瘤细胞免疫表型特征。结果:160例MM患者中CD38、CD138均为阳性,CD7、CD34、CD3、CD10均为阴性,其他抗体阳性率分别为CD56(61.5%)、CD117(18.8%)、CD19(6.5%)、CD20(12.5%)、c Kappa(71.0%)、c Lambda(76.3%)、CD81(58.1%)、CD45(46.9%)、CD27(14.3%)、CD28(17.4%)、CD33(21.4%)。CD20在小于60岁患者中的表达明显高于60岁以上患者[(78.91±16.84)%vs(38.31±11.29)%(P0.05),CD28在大于60岁患者中的表达明显高于小于60岁的患者[(70.81±19.44)%vs(43.97±11.25)%](P0.05)。CD28、CD45在女性患者中的表达明显高于男性患者[(71.75±25.20)%vs(47.25±12.04)%](P0.05)、[(74.04±19.07)%vs(48.42±25.10)%](P0.05)。CD20的表达在Ⅲ期患者明显高于Ⅰ期和Ⅱ期患者[(84.82±11.25)%vs(42.01±7.15)%](P0.05),而CD117的表达与此相反[(49.13±14.06)%vs(77.07±25.92)%](P0.05)。在女性患者中,CD81的表达在60岁以上的患者中明显升高[(43.87±24.43)%vs(74.59±22.94)%](P0.05)。在小于50岁的患者中,女性CD117的阳性率明显高于男性[(83.4±0.42)%vs(45.75±2.19)%](P0.05),而CD81的表达与此相反[(39.20±24.14)%vs(81.52±22.05)%](P0.05);在51~60岁的患者中,男性患者CD33的表达明显高于女性[(94.53±3.06)%vs(45.37±15.28)%](P0.05);在60~70岁的患者中,各分子的表达在男女之间无显著统计学差异。在大于70岁的患者中,男性CD117的表达明显高于女性[(70.40±31.40)%vs(53.50±29.98)%](P0.05),而c Kappa的表达与此相反[(32.58±13.90)%vs(76.98±22.42)%](P0.05)。结论:骨髓瘤细胞表达以CD38、CD138抗原为主,仅部分患者可伴有抗原CD56、CD117、CD19、CD20、cKappa、cLambda、CD81、CD45、CD27、CD28、CD33的表达。     

CD56、CD117表达水平与来那度胺治疗多发性骨髓瘤疗效的相关性分析

目的:探讨CD56、CD117的表达水平与来那度胺治疗多发性骨髓瘤疗效的相关性,为预测来那度胺治疗多发性骨髓瘤的疗效提供参考依据。方法:将我院2016年12月至2019年3月收治的多发性骨髓瘤患者64例,均给予来那度胺联合小剂量地塞米松治疗,所有患者根据疗效分为完全缓解(CR)、非常好的部分缓解(VGPR)、部分缓解(PR)、疾病稳定(SD)、疾病进展(PD),并且采用门诊、电话、住院等随访方式定期随访。采用流式细胞仪测定所有患者入院治疗前一天的CD56、CD117的表达水平,分析CD56、CD117表达与患者病理特征以及来那度胺疗效的相关性。结果:CD56、CD117的表达水平与多发性骨髓瘤患者的轻链分型、M蛋白分型、临床分型显著相关,与性别无关。来那度胺治疗多发性骨髓瘤的64例患者中,有效例数53例,总有效率为83%;其有效组患者中,CD56、CD117的阳性率分别为56.6%、35.8%,显著高于无效组患者(18.2%、9.1%)(P<0.05)。结论:多发性骨髓瘤患者CD56、CD117均呈现高表达的状态,其表达与患者的临床分期、M蛋白类型、轻链重链类型及来那度胺治疗的疗效有显著相关性。     

环氧化酶-2和CD44v6蛋白在宫颈鳞癌中表达及临床意义

探讨环氧化酶-2(cyclooxy-genase-2,COX-2)和CD44v6蛋白在官颈上皮内瘤样病变(CIN)和浸润宫颈鳞癌(ICC)中的表达及其意义.应用免疫组织化学方法,检测45例ICC、25例CIN和10例正常宫颈组织(NCE)中COX-2和CD44v6蛋白的表达水平,并结合临床病理特征进行分析.结果表明,在ICC、CIN和NCE中,COX-2蛋白表达阳性率分别为82.2%(37/45)、40%(10/25)和0%(0/10),差异有显著性(P＜0.05);CD44v6蛋白表达阳性率分别为88.9%(40/45)、44%(11/25)和20%(2/10),差异有显著性(P＜0.05).COX-2和CD44v6蛋白表达与ICC的分化程度和淋巴结转移相关(P＜0.05),但与临床分期无关(P＞0.05).COX-2和CD44v6可能参与了调控ICC的发生、发展过程,其高表达预示ICC预后不良.     

CD138免疫磁珠细胞分选的FISH技术在多发性骨髓瘤诊断中的应用

为了分析CD138免疫磁珠细胞分选的染色体荧光原位杂交(FISH)技术在提高多发性骨髓瘤(MM)细胞遗传学异常检测敏感性的作用。本研究选取我院收治的30例确诊MM的患者为研究对象,分离骨髓单个核细胞,应用探针组合,同时采用2种方法进行细胞遗传学检测:实验组采用CD138免疫磁珠分选浆细胞后行荧光原位杂交技术(MACS-FISH)检测;对照组直接荧光原位杂交技术(D-FISH)检测。结果:30例MM患者,实验组采用CD138 MACS-FISH检出率为83.3%,对照组D-FISH法细胞遗传异常检出率为46.7%,两组差异具有统计学意义(p<0.05)。研究结果表明:分析不同类型的细胞遗传异常,MACS-FISH法1q21检出率为46.7%,RB1检出率为50.0%,Ig H检出率为70.0%,P53检出率为20.0%;D-FISH法检出率分别为23.3%,30.0%、36.7%、10.0%。通过细胞核型分析,30例MM患者中,发现5例患者为异常核型,仅为16.7%,其中1例患者为单一结构异常,复杂异常核型患者为4例。我们的研究结论表明:进行CD138免疫磁珠分选浆细胞的FISH技术在多发性骨髓瘤诊断应用中可显著提高细胞遗传学异常检测敏感性,具有临床推广应用的价值。     

华蟾素胶囊对多发性骨髓瘤化疗患者的效果及对细胞免疫和肠道菌群的影响

目的探讨华蟾素胶囊对多发性骨髓瘤(MM)化疗患者的临床疗效和对其细胞免疫、肠道细菌的影响。方法选取95例本院收治的MM化疗患者,通过随机分组分为观察组50例和对照组45例。对照组患者常规化疗方案,观察组在其基础上给予华蟾素胶囊治疗,两组治疗时间均持续3个疗程。对比治疗后MM化疗患者临床疗效、治疗前后细胞免疫指标水平差异和肠道细菌分布情况。结果两组疗效等级分布差异显著(P0.05),观察组总有效率高于对照组(P0.05)。治疗前,两组各个细胞免疫指标水平均相近(均P0.05),治疗后观察组CD4~+/CD8~+T、DC1/DC2、CD8~+CD25~+/CD3~+T、Th1/Th2、CD4~+CD25~+/CD3~+T水平相近(P0.05),对照组各个细胞免疫指标均显著下降(均P0.05),且显著低于观察组(P0.05)。治疗前,两组Chao1指数、Shannon指数均相近(均P0.05),治疗后,观察组这两个指数均升高(P0.05),对照组均降低(P0.05)。观察组属水平相对丰度在治疗后显著升高且高于对照组(P0.05)。结论华蟾素胶囊联合化疗可以提高MM患者细胞免疫功能,改善肠道细菌平衡,具有较好的临床疗效。     

CD4+CD25+FoxP3+ 调节性T 细胞与黑龙江省HIV/AIDS 患者病情 相关性的研究

目的:比较黑龙江省HIV/AIDS患者与健康对照者(healthy controls,HCs)外周血CD4+CD25+FoxP3+调节性T细胞数量、免疫抑制功能的变化,探讨CD4+CD25+FoxP3+调节性T细胞在HIV/AIDS感染过程中的作用。方法:采用流式细胞仪检测21例HIV/AIDS患者及20例健康对照组的外周血CD4+CD25+FoxP3+调节性T细胞数量的百分比及绝对数量;采用共同培养方法检测HIV/AIDS患者外周血CD4+CD25+FoxP3+调节性T细胞免疫抑制功能的变化;实时荧光定量聚合酶链反应(RT-FQ-PCR)检测HIV/AIDS患者外周血CD4+CD25+FoxP3+调节性T细胞中FoxP3mRNA的表达。结果:黑龙江省HIV/AIDS患者外周血CD4+CD25+FoxP3+调节性T细胞比率明显高于HCs(P<0.01),而CD4+CD25+FoxP3+调节性T细胞的绝对计数显著下降,且与CD4+T细胞绝对计数成反比;混合淋巴细胞共同培养结果显示,HIV/AIDS患者外周血CD4+CD25+FoxP3+调节性T细胞的抑制功能无明显变化;HIV/AIDS患者外周血CD4+CD25+FoxP3+调节性T细胞的FoxP3 mRNA相对表达量无显著变化。结论:黑龙江省HIV/AIDS患者CD4+CD25+FoxP3+调节性T细胞的数量变化与病情相关。     

CD4+CD25+调节性T细胞在自身免疫性肝炎发病中的作用

目的:比较自身免疫性肝炎(autoimmune hepatitis,AIH)患者与健康对照者(healthy controls,HCs)外周血CD4+CD25+调节性T细胞(CD4+CD25+Tregs)数量、免疫抑制功能的变化,探讨CD4+CD25+Tregs参与AIH发病的可能机制.方法:采用流式细胞仪检测8例AIH患者及15例健康对照组的外周血CD4+CD25+Tregs数量的百分比及绝时数量;采用共同培养方法检测AIH患者外周血CD4+CD25+Tregs的免疫抑制功能的变化;实时荧光定量聚合酶链反应(RT-FQ-PCR)检删AIH患者外周血CD4+CD25+Tregs中FoxP3mRNA的表达.结果:AIH患者外周血CD4+CD25+Tregs数量明显低于HCs(p<0.01);混合淋巴细胞共同培养结果显示,AIH患者外周血CD4+CD25+Tregs抑制功能明显低于HCs组(p<0.01);AIH患者外周血CD4+CD25+Tregs的FoxP3 mRNA相对表达量显著降低,与HCs组比较有显著性差异(p<0.01).结论:CD4+CD25+Tregs细胞的数量的减少和Foxp3表达的降低所造成的CD4+CD25+Tregs细胞免疫抑制功能受损可能是AIH发病的一个因素.     

多发性骨髓瘤患者外周血淋巴细胞亚群分析及与预后因子的相关性

目的通过对多发性骨髓瘤（MM）患者外周血淋巴细胞亚群的检测以评价MM患者机体的免疫功能状态。方法采用流式细胞术检测36例MM患者和25例健康志愿者外周血T、B淋巴细胞、NK细胞及CD4＋CD25＋T细胞的表达。结果与正常对照组相比,MM患者外周血的CD4、CD19细胞的表达显著下调,CD8细胞的表达显著上调,CD4/CD8比值则显著降低（P〈0.05或〈0.01）;MM患者外周血的CD4＋CD25＋T细胞占CD3＋T细胞的比例明显增高（P〈0.01）,且与血清中的β2-MG浓度成正相关（γ=0.56,P〈0.05）。结论 MM患者体内存在淋巴细胞亚群的异常表达、CD4＋CD25＋Treg细胞的异常扩增,可能是MM患者体内广泛存在免疫缺陷的一个主要原因。     

CD30/CD30 ligand and CD40/CD40 ligand in malignant lymphoid disorders

CD30 and CD40 are members of the tumor necrosis factor (TNF) receptor family. These two receptors have pleiotropic biologic functions including induction of apoptosis and enhancing cell survival. This review will discuss the pattern of expression of these receptors in malignant lymphoid disorders and their prospective ligands. Understanding issues related to these two ligands and their receptors in lymphoid malignancies may help to improve the classification of these diseases and could open the doors for new treatment strategies.     

Follicular lymphoma intratumoral CD4+CD25+GITR+ regulatory T cells potently suppress CD3/CD28-costimulated autologous and allogeneic CD8+CD25- and CD4+CD25- T cells

Regulatory T cells (T(R)) play a critical role in the inhibition of self-reactive immune responses and as such have been implicated in the suppression of tumor-reactive effector T cells. In this study, we demonstrate that follicular lymphoma (FL)-infiltrating CD8+ and CD4+ T cells are hyporesponsive to CD3/CD28 costimulation. We further identify a population of FL-infiltrating CD4+CD25+GITR+ T(R) that are significantly overrepresented within FL nodes (FLN) compared with that seen in normal (nonmalignant, nonlymphoid hyperplastic) or reactive (nonmalignant, lymphoid hyperplastic) nodes. These T(R) actively suppress both the proliferation of autologous nodal CD8+CD25- and CD4+CD25- T cells, as well as cytokine production (IFN-gamma, TNF-alpha and IL-2), after CD3/CD28 costimulation. Removal of these cells in vitro by CD25+ magnetic bead depletion restores both the proliferation and cytokine production of the remaining T cells, demonstrating that FLN T cell hyporesponsiveness is reversible. In addition to suppressing autologous nodal T cells, these T(R) are also capable of suppressing the proliferation of allogeneic CD8+CD25- and CD4+CD25- T cells from normal lymph nodes as well as normal donor PBL, regardless of very robust stimulation of the target cells with plate-bound anti-CD3 and anti-CD28 Abs. The allogeneic suppression is not reciprocal, as equivalent numbers of CD25+FOXP3+ cells derived from either normal lymph nodes or PBL are not capable of suppressing allogeneic CD8+CD25- and CD4+CD25- T cells, suggesting that FLN T(R) are more suppressive than those derived from nonmalignant sources. Lastly, we demonstrate that inhibition of TGF-beta signaling partially restores FLN T cell proliferation suggesting a mechanistic role for TGF-beta in FLN T(R)-mediated suppression.     

CD154 (CD40 ligand)

CD40 ligand, a type II transmembrane protein recently renamed CD154, was originally considered restricted to activated T lymphocytes, functioning as a mediator of T cell-dependent B cell activation, proliferation, and differentiation. However, the spectrum of CD154 expression and function has broadened considerably during recent years, establishing new roles as a central mediator of immunity and inflammation for this member of the tumor necrosis factor (TNF) gene superfamily. The emerging picture indicates that ligation of the receptor CD40 via CD154, most potently in its trimeric form, functions in two ways. CD154 modulates physiologic processes, such as T cell-mediated effector functions and general immune responses required for appropriate host defense, but also triggers the expression of pro-inflammatory mediators, such as cytokines, adhesion molecules, and matrix degrading activities, all of which are associated with the pathogenesis of chronic inflammatory diseases, e.g., autoimmune disorders, arthritis, atherosclerosis, and cancer. Accordingly, CD40/CD154 interactions have advanced as a potential therapeutic target for these diseases, whereby two opposing strategies, interruption as well as enhancement of CD40 signaling, are explored for beneficial outcomes.     

CD30/CD30 ligand (CD153) interaction regulates CD4+ T cell-mediated graft-versus-host disease

CD30, a TNFR family member, is expressed on activated CD4(+) and CD8(+) T cells and B cells and is a marker of Hodgkin's lymphoma; its ligand, CD30L (CD153) is expressed by activated CD4(+) and CD8(+) T cells, B cells, and macrophages. Signaling via CD30 can lead to proliferation or cell death. CD30-deficient (-/-) mice have impaired thymic negative selection and increased autoreactivity. Although human alloreactive T cells preferentially reside within the CD30(+) T cell subset, implicating CD30 as a regulator of T cell immune responses, the role of CD30/CD153 in regulating graft-vs-host disease (GVHD) has not been reported. We used a neutralizing anti-CD153 mAb, CD30(-/-) donor mice, and generated CD153(-/-) recipient mice to analyze the effect of CD30/CD153 interaction on GVHD induction. Our data indicate that the CD30/CD153 pathway is a potent regulator of CD4(+), but not CD8(+), T cell-mediated GVHD. Although blocking CD30/CD153 interactions in vivo did not affect alloreactive CD4(+) T cell proliferation or apoptosis, a substantial reduction in donor CD4(+) T cell migration into the gastrointestinal tract was readily observed with lesser effects in other GVHD target organs. Blockade of the CD30/CD153 pathway represents a new approach for preventing CD4(+) T cell-mediated GVHD.     

CD134L engagement enhances human B cell Ig production: CD154/CD40, CD70/CD27, and CD134/CD134L interactions coordinately regulate T cell-dependent B cell responses

CD134 is a member of the TNFR family expressed on activated T cells, whose ligand, CD134L, is found preferentially on activated B cells. We have previously reported that the CD70/CD27 interaction may be more important in the induction of plasma cell differentiation after the expansion phase induced by the CD154/CD40 interaction has occurred. When CD134-transfected cells were added to PBMCs stimulated with pokeweed mitogen, IgG production was enhanced in a dose-dependent fashion. Addition of CD134-transfected cells to B cells stimulated with Staphylococcus aureus Cowan I strain/IL-2 resulted in little if any enhancement of B cell IgG production and proliferation. We found that while CD134-transfected cells induced no IgG production by themselves, it greatly enhanced IgG production in the presence of CD40 stimulation or T cell cytokines such as IL-4 and IL-10. The addition of CD134-transfected cells showed only a slight increase in the number of plasma cells compared with that in the culture without them, indicating that an increased Ig production rate per cell is responsible for the observed enhancing effect of CD134L engagement rather than increase in plasma cell generation. These results strongly suggest different and sequential roles of the TNF/TNFR family molecules in human T cell-dependent B cell responses through cell-cell contacts and the cytokine network.     

Mesenchymal stromal cells impair the differentiation of CD14(++) CD16(-) CD64(+) classical monocytes into CD14(++) CD16(+) CD64(++) activate monocytes

Background aimsMesenchymal stromal cells (MSC) possess immunomodulatory activity both in vitro and in vivo. However, little information is available regarding their function during the initiation of immunologic responses through their interactions with monocytes. While many studies have shown that MSC impair the differentiation of monocytes into dendritic cells and macrophages, there are few articles showing the interaction between MSC and monocytes and none of them has addressed the question of monocyte subset modulationMethodsTo understand better the mechanism behind the benefit of MSC infusion for graft-versus-host treatment through monocyte involvement, we performed mixed leucocyte reactions (MLR) in the presence and absence of MSC. After 3 and 7 days, cultures were analyzed by flow cytometry using different approachesResultsMSC induced changes in monocyte phenotype in an MLR. This alteration was accompanied by an increase in monocyte counting and CD14 expression. MSC induced monocyte alterations even without contact, although the parameters above were more pronounced with cell–cell contact. Moreover, the presence of MSC impaired major histocompatibility complex (MHC) I and II, CD11c and CCR5 expression and induced CD14 and CD64 expression on monocytes. These alterations were accompanied by a decrease in interleukin (IL)-1β and IL-6 production by these monocytes, but no change was observed taking into account the phagocytosis capacity of these monocytesConclusionsOur results suggest that MSC impair the differentiation of CD14⁺⁺ CD16^? CD64⁺ classical monocytes into CD14⁺⁺ CD16⁺ CD64⁺⁺ activated monocytes, having an even earlier role than the differentiation of monocytes into dendritic cells and macrophages.     

Requirement of oxidation-dependent CD40 homodimers for CD154/CD40 bidirectional signaling

It is well established that the CD154/CD40 interaction is required for T cell-dependent B cell differentiation and maturation. However, the early molecular and structural mechanisms that orchestrate CD154 and CD40 signaling at the T cell/APC contact site are not well understood. We demonstrated that CD40 engagement induces the formation of disulfide-linked (dl) CD40 homodimers that predominantly associate with detergent-resistant membrane microdomains. Mutagenesis and biochemical analyses revealed that (a) the integrity of the detergent-resistant membranes is necessary for dl-CD40 homodimer formation, (b) the cytoplasmic Cys(238) of CD40 is the target for the de novo disulfide oxidation induced by receptor oligomerization, and (c) dl-CD40 homodimer formation is required for CD40-induced interleukin-8 secretion. Stimulation of CD154-positive T cells with staphylococcal enterotoxin E superantigen that mimics nominal antigen in initiating cognate T cell/APC interaction revealed that dl-CD40 homodimer formation is required for interleukin-2 production by T cells. These findings indicate that dl-CD40 homodimer formation has a physiological role in regulating bidirectional signaling.     

A ligand for CD5 is CD5

Recognition by scavenger receptor cysteine-rich domains on membrane proteins regulates innate and adaptive immune responses. Two receptors expressed primarily on T cells, CD5 and CD6, are linked genetically and are structurally similar, both containing three scavenger receptor cysteine-rich domains in their extracellular regions. A specific cell surface interaction for CD5 has been difficult to define at the molecular level because of the susceptibility of CD5 protein to denaturation. By using soluble CD5 purified at neutral pH to preserve biological activity, we show that CD5 mediates species-specific homophilic interactions. CD5 domain 1 only is involved in the interaction. CD5 mAbs that have functional effects in humans, rats, and mice block homophilic binding. Ag-specific responses by mouse T cells in vitro were increased when engagement of human CD5 domain 1 was inhibited by mutation or by IgG or Fab fragment from a CD5 mAb. This showed that homophilic binding results in productive engagement. Enhancement of polyclonal immune responses of rat lymph node cells by a Fab fragment from a CD5 mAb shown to block homophilic interactions provided evidence that the extracellular region of CD5 regulates inhibition in normal cells. These biochemical and in vitro functional assays provide evidence that the extracellular region of CD5 regulates immunity through species-specific homophilic interactions.