双特异性单抗对胃肠癌的体内杀伤作用

目的探讨抗CD3抗CEA双特异性单链抗体介导CIK细胞在体内杀伤胃肠癌的作用。方法分别将人胃低分化腺癌BGC823细胞和人结肠中分化腺癌SW1116细胞种植裸鼠后,建立裸鼠胃癌及肠癌模型,分为3组分别经尾静脉注入生理盐水、CIK细胞及CIK细胞＋双特异性单抗,每3天治疗1次,共6次,取出肿瘤组织称重,并计算出各组的抑瘤率。结果在体内,CIK组和CIK＋双抗组均可抑制肿瘤生长,对胃癌的抑瘤率分别为29.90%和44.33%。对结肠癌的抑瘤率分别为14.93%和38.81%。CIK＋双抗组：胃癌及结肠癌的瘤体积、瘤重明显小于CIK组。结论 CIK细胞本身可以抑制肿瘤细胞的生长,在加入抗CD3抗CEA双特异性单链抗体后抑瘤作用明显增强。     

Surface markers of cloned human T cells with various cytolytic activities

Human T cells stimulated in secondary allogeneic mixed lymphocyte culture (MLC) were cloned under limiting conditions in microculture systems using T cell growth factor and irradiated allogeneic cells. Clones with lytic activity against either phytohemagglutinin-induced blast cells bearing the stimulating alloantigen(s) (cytotoxic T lymphocyte [CTL] activity), L1210 mouse lymphoma cells coated with rabbit antibody (antibody-dependent cell-mediated cytotoxicity [ADCC]), or K562 human target cells were selected, expanded, and then analyzed for different surface markers, including rosette formation with sheep erythrocytes (E rosettes), receptors for the fc portion of IgG or IgM (Fc gamma R and Fc mu R), and a group of antigens recognized by monoclonal antibodies including Ia, 4F2, OKT8,a nd OKT4. All the cytotoxic cells were E rosette+, Ia+ and 4f2+. Expression of Fc gamma R was restricted to the clones active in ADCC. CTL clones were either OKT8+ or OKT8-. Furthermore, three of the OKT8- CTL clones were OKT4+. In addition, some cytolytic clones devoid of specific CTL activity were OKT8+. It thus appears that the claim that human CTL are OKT8+, OKT4-, and Ia- is not supported by the analysis of their phenotype at the clonal level.     

Development of humanized bispecific antibodies reactive with cytotoxic lymphocytes and tumor cells overexpressing the HER2 protooncogene

The HER2 protooncogene encodes a 185-kD transmembrane phosphoglycoproteins, human epidermal growth factor receptor 2 (p185HER2), whose amplified expression on the cell surface can lead to malignant transformation. Overexpression of HER2/p185HER2 is strongly correlated with progression of human ovarian and breast carcinomas. Recent studies have shown that human T cells can be targeted with bispecific antibody to react against human tumor cells in vitro. We have developed a bispecific F(ab')2 antibody molecule consisting of a humanized arm with a specificity to p185HER2 linked to another arm derived from a murine anti-CD3 monoclonal antibody that we have cloned from UCHT1 hybridoma. The antigen-binding loops for the anti-CD3 were installed in the context of human variable region framework residues, thus forming a fully humanized BsF(ab')2 fragment. Additional variants were produced by replacement of amino acid residues located in light chain complementarity determining region 2 and heavy chain framework region 3 of the humanized anti-CD3 arm. Flow cytometry analysis showed that the bispecific F(ab')2 molecules can bind specifically to cells overexpressing p185HER2 and to normal human peripheral blood mononuclear cells bearing the CD3 surface marker. In additional experiments, the presence of bispecific F(ab')2 caused up to fourfold enhancement in the cytotoxic activities of human T cells against tumor cells overexpressing p185HER2 as determined by a 51Cr release assay. These bispecific molecules have a potential use as therapeutic agents for the treatment of cancer.     

转mdr1基因诱导CIK细胞耐药并保持肿瘤杀伤活性

目的　将多药耐药基因 (mdr1)转入细胞因子诱导的杀伤 (cytokine inducedkiller,CIK)细胞 ,观察其是否产生耐药性并且保持肿瘤杀伤活性。方法　用IFN γ ,CD3单抗 ,IL 2 ,IL 1等细胞因子体外诱导外周血单个核细胞获得CIK细胞。采用电穿孔方法将mdr1基因表达质粒pHamdr转入CIK细胞。转染后 72h提取细胞总RNA ,DNA酶消化残留质粒DNA后进行RT PCR ,鉴定mdr1基因表达 ;流式细胞仪检测细胞膜耐药蛋白 (P gp)表达。四唑蓝比色法 (MTT)检测转基因后CIK细胞对阿霉素和秋水仙碱的耐药性 ;同时检测转染前后CIK细胞对MCF7细胞 (人类乳腺癌细胞系 )的杀伤活性变化。结果　转染mdr1后的CIK细胞mdr1mRNA阳性 ,P gp阳性的CIK细胞为 2 1%～ 37% (平均 2 7% )。转染后CIK细胞获得了多药耐药性 ,对阿霉素的IC50 值较转染前升高了 2 2 .3～ 4 5 .8倍 ,对秋水仙碱的IC50 值升高了 6 .7～ 11.35倍。转染前后CIK细胞对MCF7肿瘤细胞的杀伤活性无明显变化 (P >0 0 5 )。结论　CIK细胞转入mdr1基因后获得了多药耐药性 ,转染后的CIK细胞仍然保持原有的肿瘤细胞杀伤活性。     

细胞因子诱导的杀伤细胞体外培养的初步研究(英文)

Objective To investigate the culture method of cytokines induced killer cells using CD3 monoclonal antibody,IL-2,IFN-γ and IL-1α in vitro.Methods Peripheral blood mononuclear cells (PBMC) were collected by a blood cell separator from 8 patients with refractory lymphoma,then expanded by priming them with recombinant IFN-γ,monoclonal antibody (mAb) to CD3 and human recombinant IL-1α,followed by adding recombinant IL-2 the next day.The CIK cells were counted on d1,4,7,10,13 of incubation,respectively.The phenotypic patterns were characterized before culture and on d13 of culture.Results CIK cells colony formed on d3.On d13,the CIK cells counting reached up to (7-18)×109(mean 12.7×109),multiplying 44-140 times(mean 98 times).Cell survival rate was over 90%.The average percentage of cells expressing CD3+,CD4+,CD8+ and CD3+ CD56+ were also increased from (50.9±3.5)%,(29.9±1.7)%,(41.3±3.2)%,(1.6±0.2)% to (90.2±1.6)%,(40.6±5.5)%,(52.8±4.9)% and (33.1±4.0)%,respectively.Conclusions CIK cells developed by the culture method have high in vitro proliferate rate and tumor-killing capacity.The simplicity of the operation and autogenic source of the cells indicate the method could be applied clinically treating patients with refractory lymphoma.     

Activation of human T cells by FcR nonbinding anti-CD3 mAb,hOKT3gamma1(Ala-Ala)

Dimeric Fc receptor (FcR) nonbinding anti-CD3 antibodies have been developed to minimize toxicities associated with classical anti-CD3 monoclonal antibodies (e.g., OKT3). Studies with murine analogs of non-FcR-binding antibodies have shown reduced mitogenicity compared to OKT3. In a trial of an FcR nonbinding humanized anti-CD3 mAb hOKT3gamma1(Ala-Ala) for treatment of patients with type 1 diabetes, we found significant increases in IL-10 and IL-5 in the serum of 63% and 72% of patients, respectively, and TNF-alpha and IL-6 levels that were lower than those previously reported following OKT3 therapy. The activation signal delivered by hOKT3gamma1(Ala-Ala) was associated with calcium signaling and cytokine production by previously activated human cells in vitro. However, the production of IL-10, compared to IFN-gamma on a molar basis, was greater after culture with hOKT3gamma1(Ala-Ala) than with OKT3. Flow cytometric studies confirmed that OKT3 induced IFN-gamma and IL-10 production, but hOKT3gamma1(Ala-Ala) induced only detectable IL-10 production in CD45RO(+) cells. Moreover, in vivo, we found IL-10(+)CD4(+) T cells after drug treatment. These cells were heterogeneous but generally CD45RO(+), CTLA-4(-), and expressed CCR4. A subgroup of these cells expressed TGF-beta. Thus, the non-FcR binding anti-CD3 mAb, hOKT3gamma1(Ala-Ala) delivers an activation signal to T cells that is quantitatively and qualitatively different from OKT3. It leads to the generation of T cells that might inhibit the autoimmune response and may be involved in the beneficial effect on beta cell destruction in Type 1 diabetes.     

MT-103 Micromet/MedImmune

Micromet AG and Medlmmune Inc are developing MT-103, a single-chain bispecific recombinant antibody from Micromet's BiTE (bispecific T-cell engager) product platform that binds both the CD19 antigen and the T-cell receptor (CD3), for the potential treatment of B-cell lymphoma. The company is also investigating the compound for the potential treatment of chronic lymphocytic leukemia and acute lymphoblastic leukemia.     

CD28 interaction with B7 costimulates primary allogeneic proliferative responses and cytotoxicity mediated by small, resting T lymphocytes

Engagement of the CD3/T cell antigen receptor complex on small, resting T cells is insufficient to trigger cell-mediated cytotoxicity or to induce a proliferative response. In the present study, we have used genetic transfection to demonstrate that interaction of the B7-BB1 B cell activation antigen with the CD28 T cell differentiation antigen costimulates cell-mediated cytotoxicity and proliferation initiated by either anti-CD2 or anti-CD3 monoclonal antibody (mAb). Moreover, a B7-negative Burkitt's lymphoma cell line that fails to stimulate an allogeneic mixed lymphocyte response is rendered a potent stimulator after transfection with B7. The mixed leukocyte reaction proliferative response against the B7 transfectant is inhibited by either anti-CD28 or B7 mAb. We also demonstrate that freshly isolated small, resting human T cells can mediate anti-CD3 or anti-CD2 mAb-redirected cytotoxicity against a murine Fc receptor-bearing mastocytoma transfected with human B7. These preexisting cytotoxic T lymphocytes in peripheral blood are present in both the CD4 and CD8 subsets, but are preferentially within the CD45RO+ "memory" population. While small, resting T cells apparently require costimulation by CD28/B7 interactions, this requirement is lost after T cell activation. Anti-CD3 initiates a cytotoxic response mediated by in vitro cultured T cell clones in the absence of B7 ligand. The existence of functional cytolytic T cells in the small, resting T cell population may be advantageous in facilitating rapid responses to immune challenge.     

Evidence by reactivity with hybridoma antibodies for a probable myeloid origin of peripheral blood cells active in natural cytotoxicity and antibody-dependent cell-mediated cytotoxicity.

Lymphocytes with Fc receptors (FcR) for IgG active in natural cytotoxicity and antibody-dependent cellular cytotoxicity were separated into sheep erythrocyte rosetting (E+) and nonrosetting (E-) fractions, and examined for reactivity with the OK panel of hybridoma-produced monoclonal antibodies. Few cells in either the E+ FcR+ or the E- FcR+ fraction reacted with seven antibodies used to define T cells in various stages of differentiation (OK3, OKT4, OKT5, OKT6, OKT8, OKT9, OKT10). Neither fraction expressed an Ia-like antigen (detected by OKI1), but both were highly reactive with OKM1, an antibody that reacts with monocytes and granulocytes. Incubation of these cytotoxic effector cells with OKM1 plus complement abolished all cytotoxic reactivity, but incubation with a pan-T cell antibody (OKT3) plus complement had no significant effect. These cells were not monocyte precursors, because they could not be induced in vitro to develop macrophage characteristics. The data indicate that most cytotoxic effector cells in natural cytotoxicity and antibody-dependent cellular cytotoxicity are not in the T cell lineage, but have a myeloid origin.     

Bispecific single-chain diabody-mediated killing of endoglin-positive endothelial cells by cytotoxic T lymphocytes

We present a novel vascular tumor therapy approach based on lysing endothelial cells by cytotoxic T lymphocytes (CTLs). Retargeting of CTLs is achieved by a recombinant bispecific antibody molecule (bispecific single-chain diabody) directed against human endoglin (CD105, EDG) and the T-cell coreceptor CD3 (scDb EDGCD3). Bacterially expressed scDb EDGCD3 was able to bind to endoglin-expressing endothelial cells as well as CD3-expressing T lymphocytes. The single-chain diabody mediated killing of endothelial cells (HUVEC, HMEC) by activated cytotoxic T lymphocytes at picomolar concentrations, and cells not expressing endoglin were not affected. Because endoglin is up-regulated in the vasculature of many solid tumors, this antibody molecule should be capable of lysing tumor endothelial cells and thus destroying the vascular bed of the tumor.     

包被抗人CD3单克隆抗体对CIK细胞生长影响的研究

目的 通过对比包被和未包被抗人CD3单克隆抗体培养瓶中的杀伤细胞(CIK细胞)生长状态及培养一段时间后收集的细胞数,研究在临床常用培养CIK细胞的方法上,后续增加对培养瓶进行包被抗人CD3单克隆抗体的预处理后,对CIK细胞生长的影响.方法 提取健康者的外周血单个核细胞(PBMC),无血清培养液调整好细胞浓度,在75 cm2密封盖培养瓶中等量加入包被和未包被抗人CD3抗体,以及IFN-γ.培养24 h后加入CIK混合刺激因子,以后每天观察细胞的生长情况,根据细胞的生长情况进行传代扩增(扩增至未包被的培养瓶中),第12天收集细胞进行比较.结果 包被抗人CD3抗体的75 cm2密封盖培养瓶培养的CIK细胞,在培养第3天即可进行传代扩增,以后每天都可进行扩增;用未包被抗人CD3抗体75 cm2密封盖培养瓶培养的CIK细胞在第5天才可进行传代扩增,以后每隔1 d可以进行扩增,第12天收集的细胞总数未包被的为1.2×109,包被的为3.3×109.结论 包被细胞因子抗人CD3单克隆抗体对正常条件培养的CIK细胞生长状态和较短时间内收获的CIK细胞总量有很好的正影响.     

自体CIK细胞联合IL-2治疗老年人B细胞性恶性淋巴瘤的临床研究

本研究评价自体CIK细胞联合IL-2治疗老年人B细胞性恶性淋巴瘤的有效性和安全性。采集9例老年B细胞性恶性淋巴瘤患者外周血单个核细胞,在体外经干扰素-γ(IFN-γ)、白介素-2(IL-2)、抗CD3单克隆抗体诱导成CIK细胞,回输细胞数为(2-3)×109个,回输后应用IL-2 100万U/day,皮下注射,第1-10天。28天为1个疗程,共完成了64个周期的自体CIK细胞输注。观察治疗前后细胞免疫功能、肿瘤相关生物学指标、影像学特征、疾病缓解情况、生活质量及生存期的变化。结果表明,7例接受8个周期的CIK细胞输注的患者和2例接受4个周期的输注的患者均未出现不良反应。CIK细胞治疗后CD3+、CD3+CD8+、CD3+CD56+细胞比例明显升高(p0.05),β2微球蛋白、LDH水平显著下降(p0.05)。所有患者症状改善、生活质量显著提高(p0.01)。8例达完全缓解,1例完成8周期的CIK细胞输注后曾一度达良好的部分缓解,但因上呼吸道感染并发急性大面积心肌梗死和淋巴瘤持续进展而死亡。结论:自体CIK细胞联合IL-2治疗老年人B细胞性恶性淋巴瘤安全有效。     

Constitutive expression of pore-forming protein in peripheral blood gamma/delta T cells: implication for their cytotoxic role in vivo

The cytotoxic activity and pore-forming protein (PFP) expression of human peripheral blood (PB) gamma/delta T cells were examined. Fresh gamma/delta T cells isolated from PB lymphocytes by fluorescence-activated cell sorting exhibited a substantial natural killer-like cytotoxic activity against K562 target cells and had a high cytotoxic potential triggered by anti-CD3 monoclonal antibody (mAb) against P815 target cells bearing Fc gamma R. Immunocytochemical staining with an anti-PFP mAb revealed that virtually all PB gamma/delta T cells are granular lymphocytes with abundant PFP in their cytoplasmic granules. Constitutive expression of PFP in PB gamma/delta T cells was also demonstrated by Northern blot analysis. These observations support the proposed role of gamma/delta T cells in cytolytic immune surveillance in vivo.     

Generation of an HLA-restricted cytotoxic T cell line reactive against cultured tumor cells from a patient infected with human T cell leukemia/lymphoma virus

Lymphocytes from a patient who had an unusually long survival after therapy for a human T cell leukemia/lymphoma virus (HTLV)-associated T cell lymphoma were stimulated in vitro with an autologous tumor cell line, and the generation of cytotoxic T lymphocytes (CTL) was studied. CTL generated were directed against autologous (HTLV-associated tumor cells. These propagated CTL were OKT3+, OKT4-, and OKT8+. The cytotoxic activity required target tumor cells that were infected with HTLV and also expressed histocompatibility antigens in common with the patient, suggesting a major histocompatibility complex-restricted associative recognition of target antigens expressed on the tumor cell membrane.     

Redirected lysis of human melanoma cells by a MCSP/CD3-bispecific BiTE antibody that engages patient-derived T cells

Melanoma-associated chondroitin sulfate proteoglycan (MCSP; also called HMW-MAA, CSPG4, NG2, MSK16, MCSPG, MEL-CSPG, or gp240) is a well characterized melanoma cell-surface antigen. In this study, a new bispecific T-cell engaging (BiTE) antibody that binds to MCSP and human CD3 (MCSP-BiTE) was tested for its cytotoxic activity against human melanoma cell lines. When unstimulated peripheral mononuclear blood cells (PBMCs) derived from healthy donors were cocultured with melanoma cells at effector:target ratios of 1:1, 1:5, or 1:10, and treated with MCSP-BiTE antibody at doses of 10, 100, or 1000 ng/mL, all MCSP-expressing melanoma cell lines (n=23) were lysed in a dose-dependent and effector:target ratio-dependent manner, whereas there was no cytotoxic activity against MCSP-negative melanoma cell lines (n=2). To investigate whether T cells from melanoma patients could act as effector cells, we cocultured unstimulated PBMCs with allogeneic melanoma cells from 13 patients (4 stage I/II, 3 stage III, and 6 stage IV) or with autologous melanoma cells from 2 patients (stage IV). Although cytotoxic activity varied, all 15 PBMC samples mediated significant redirected lysis by the BiTE antibody. When PBMC or CD8 T cells were prestimulated by anti-CD3 antibody OKT-3 and interleukin-2, the MCSP-BiTE concentrations needed for melanoma cell lysis decreased up to 1000-fold. As MCSP is expressed on most human melanomas, immunotherapy with MCSP/CD3-bispecific antibodies merits clinical investigation.     

Anti-CD70 antibodies: a potential treatment for EBV+ CD70-expressing lymphomas

A monoclonal antibody (Rituximab) directed against the B-cell surface antigen, CD20, is increasingly used as a therapy for B-cell lymphomas. However, CD20 is expressed on all normal mature B cells and hence is not a specific tumor target. In contrast, CD70 is expressed on highly activated lymphocytes as well as on many B-cell and T-cell lymphomas but is not expressed on the great majority of B cells and T cells. In this report, we have explored the potential utility of anti-CD70 monoclonal antibodies for treatment of CD70+ EBV+ B-cell lymphomas. Using two Burkitt's lymphoma lines (Raji and Jijoye) that express surface CD70 and a CD70- Burkitt's lymphoma line (Akata), we show that two different monoclonal antibodies directed against human CD70 allow rabbit and human complement to kill EBV+ B cells in a CD70-dependent manner in vitro. In the absence of complement, neither anti-CD70 antibody induced in vitro killing of CD70+ cell lines. Importantly, i.p. injection of anti-CD70 antibodies also inhibited the growth of CD70+ Burkitt's lymphoma cells in severe combined immunodeficient mice but did not inhibit the growth of CD70- Burkitt's lymphoma cells. These results suggest that anti-CD70 antibodies may be useful for the treatment of CD70+ B-cell lymphomas.     

自体 CIK 细胞联合化疗治疗老年急性髓细胞白血病的临床研究简

本研究旨在评价自体CIK细胞联合化疗治疗老年人急性白血病的有效性和安全性.采集5例老年急性白血病患者外周血单个核细胞,在体外经干扰素-γ（IFN-γ）、白介素-2（IL-2）、抗CD3单克隆抗体诱导成CIK细胞,回输至患者体内,28 d为1个疗程.观察治疗前后细胞免疫功能、感染发生、血红蛋白和输血依赖以及对自然病程的影响.结果表明：5例患者共接受46个周期的CIK细胞输注治疗,回输后所有患者均未出现不良反应;CIK细胞治疗后CD3＋、CD3＋ CD8＋、CD3＋ CD56＋细胞比例显著增高（P ＜0.05）;CIK细胞治疗可以有效减少AML患者的感染发生（P＜0.05）,缩短高热持续时间（P＜0.05）;在疾病稳定期,CIK细胞输注可以减少患者红细胞的输注量（P＜0.05）,稳定血红蛋白水平;CIK细胞输注虽不能改变病程的转归,但在患者疾病进展期以及终末期,采用化疗联合CIK治疗可使患者病情一度平稳,延长生存时间.结论：自体CIK细胞联合化疗对本组5例老年急性髓系白血病安全有效.     

树突细胞与细胞因子诱导的杀伤细胞共培养增强其体内外抗肿瘤活性

目的探讨与树突细胞(DC)共培养能否增强正常人细胞因子诱导的杀伤细胞(CIK)的体内外抗瘤活性.方法分别按照常规方法从正常人外周血单个核细胞诱导DC和CIK细胞,将NB4白血病细胞冻融物(LCL)冲击或未冲击的DC与CIK细胞共培养(LCL-DC+CIK、DC+CIK),以CIK细胞单独培养作为对照.用流式细胞术分析细胞表型,酶联免疫斑点法(ELISPOT)测定分泌IFN-γ的细胞数,51Cr释放实验测定体外细胞毒活性,同时用NB4细胞系建立荷瘤裸鼠模型研究其体内抗肿瘤活性和归巢情况.结果在培养第15天, LCL-DC+CIK与CIK细胞单独培养相比,增殖速率明显提高 [(18.2±2.1)倍vs(11.6±2.3)倍, P<0.05],CD3+CD56+ 表达水平也明显提高 [(51.05±2.63)% vs(30.18±1.45)%, P<0.05],分泌IFN-γ的细胞数量明显增高 [(86.33±5.51)/104细胞 vs(44.61±3.05)/104细胞, P<0.05],同时LCL-DC+CIK对NB4、K562、KG1a的体外细胞毒活性增强.体内实验显示与单独培养CIK细胞相比,LCL-DC+CIK细胞共培养后,可明显抑制接种瘤细胞裸鼠的成瘤率,提高裸鼠的长期无瘤存活率(100% vs 66.7%, P<0.05),以DiI标记的LCL-DC+CIK细胞在接种后7 d内可在脾脏、淋巴结及肿瘤局部被检测到.LCL-DC+CIK与DC+CIK细胞在抗瘤效应上没有明显差异.结论与DC共培养可使CIK细胞获得更高的增殖速率和更强的体内外抗肿瘤活性,可以作为一种临床有效的抗白血病免疫治疗策略.     

Induction of specific nonresponsiveness in unprimed human T cells by anti-CD3 antibody and alloantigen

Fresh peripheral blood mononuclear cells exposed to alloantigen for 3-8 d in the presence of anti-CD3 antibodies showed no response after restimulation with cells from the original donor but remained capable of responding to third-party donors. Antigen-specific nonresponsiveness was induced by both nonmitogenic and mitogenic anti-CD3 antibodies but not by antibodies against CD2, CD4, CD5, CD8, CD18, or CD28. Nonresponsiveness induced by anti-CD3 antibody in mixed leukocyte culture was sustained for at least 34 d from initiation of the culture and 26 d after removal of the antibody. Anti-CD3 antibody also induced antigen-specific nonresponsiveness in cytotoxic T cell generation assays. Anti-CD3 antibody did not induce nonresponsiveness in previously primed cells. Nonresponsiveness induced by anti-CD3 did not appear to be associated with suppressor cell activation. Thus, co-stimulation of the T cell receptor-CD3 complex on unprimed T cells with a fluid phase anti-CD3 antibody and allogenic major histocompatibility complex antigens can induce either clonal anergy or clonal deletion. These results suggest novel approaches for achieving transplantation tolerance.     

Dramatic rise in plasma viremia after CD8(+) T cell depletion in simian immunodeficiency virus-infected macaques

To determine the role of CD8(+) T cells in controlling simian immunodeficiency virus (SIV) replication in vivo, we examined the effect of depleting this cell population using an anti-CD8 monoclonal antibody, OKT8F. There was on average a 99.9% reduction of CD8 cells in peripheral blood in six infected Macaca mulatta treated with OKT8F. The apparent CD8 depletion started 1 h after antibody administration, and low CD8 levels were maintained until day 8. An increase in plasma viremia of one to three orders of magnitude was observed in five of the six macaques. The injection of a control antibody to an infected macaque did not induce a sustained viral load increase, nor did it significantly reduce the number of CD8(+) T cells. These results demonstrate that CD8 cells play a crucial role in suppressing SIV replication in vivo.