Th17和Tc17在肿瘤微环境中的研究进展

总结近期国内外关于肿瘤浸润的淋巴细胞（TIL）中CD4+Th17细胞及CD8+Tc17细胞的研究现状。运用检索PubMed检索系统,以Th17细胞、Tc17细胞、肿瘤微环境等为关键词,检索2000年1月-2013年12月的相关文献,共检索到到Th17与肿瘤微环境相关英文文献 71条,Tc17细胞与肿瘤微环境相关3篇。通过国内外大量的动物模型和体内外实验证明了Th17和Tc17有其独特的调节、分化和可塑性等特点,并且通过与肿瘤微环境中其它免疫细胞的相互作用,Th17与Treg的失衡及Tc17与MDSC等相互作用机制,在肿瘤微环境中起着重要作用。Th17/Tc17是独特的T淋巴细胞,对肿瘤发生、发展、乃至预后均密切相关,将为抗肿瘤免疫治疗提供新的有效治疗方案。     

An essential role of antigen-presenting cell/T-helper type 1 cell-cell interactions in draining lymph node during complete eradication of class II-negative tumor tissue by T-helper type 1 cell therapy

Prior studies have shown that transfer of ovalbumin (OVA)-specific T helper type 1 (Th1) cells into mice bearing MHC class II+ OVA-expressing tumor cells (A20-OVA) causes complete tumor rejection. Here we show that, although Th1 cell therapy alone was not effective against MHC class II- OVA-expressing tumor cells (EG-7), treatment of mice bearing established EG-7 tumors by i.v. transfer of Th1 cells combined with i.t. injection of the model tumor antigen OVA induced complete tumor rejection. Transferred Th1 cells enhanced the migration of tumor-infiltrating antigen-presenting cells (APC) that had processed OVA into the draining lymph node (DLN). Although transferred Th1 cells were randomly distributed in DLN, distal LN, spleen, and tumor tissue, active proliferation of Th1 cells always initiated in DLN, where Th1 cells efficiently interacted with APC that presented OVA. In parallel, OVA-tetramer+ CTLs, showing EG-7-specific cytotoxicity, were highly induced in DLN and the local tumor site. The OVA-tetramer+ CTL functioned systemically because two bilateral tumor masses were both completely rejected on treatment of one tumor. Furthermore, either active proliferation of transferred Th1 cells or generation of tetramer+ CTL was not induced in MHC class II-deficient mice and LN-deficient Aly/Aly mice. These results indicate that DLN is an indispensable organ for initiating active APC/Th1 cell interactions, which is critical for inducing complete eradication of tumor mass by tumor-specific CTL.     

Interleukin-4 and CpG oligonucleotide therapy suppresses the outgrowth of tumors by activating tumor-specific Th1-type immune responses

Because IL-4 and CpG oligodeoxynucleotides (CpG-ODNs) are immune stimulants, we evaluated the antitumor effects of IL-4 gene therapy and CpG-ODN treatment in a poorly immunogenic murine cancer model. We used a murine colorectal cancer MC38 cell line overexpressing the IL-4 gene (MC38-IL4). Incubation with MC38-IL4 and CpG-ODN enhanced bone marrow-derived dendritic cell (DC) maturation in vitro. In addition, interferon (IFN)-γ production was significantly increased in na?ve splenocytes after they were coincubated with MC38-IL4 and CpG-ODN. When mice bearing MC38 wild-type tumors were inoculated subcutaneously with MC38-IL4 cells and CpG-ODN, the outgrowth of established parental tumors was significantly suppressed compared to those in the MC38-IL4-treated group (IL-4 vs. IL-4 + CpG-ODN, p=0.015). A marked infiltration of CD8+ cells in the established parental tumors of mice treated with MC38-IL4 and CpG-ODN was confirmed by immunohistochemical analyses (MC38-IL4, 2.8 ± 1.9 cells/field vs. MC38-IL4 + CpG-ODN, 20.7 ± 15.3 cells/field, p=0.027). Significant tumor-specific cytolysis was detected when splenocytes of MC38-IL4 + CpG-ODN-treated mice were stimulated by γ-irradiated MC38-IL4 cells and CpG-ODN twice weekly in vitro and used as effector cells in a chromium-release assay (32.2 ± 3.5% for MC38 cells vs. 3.2 ± 1.1% for YAC-1 cells; at an effector to target ratio of 40). These results suggest that IL-4 and CpG-ODN treatment promotes potent Th1-type antitumor immune responses. Therefore, the combination of IL-4 gene therapy and CpG-ODN treatment for cancer should be evaluated in clinical trials.     

The critical role of Th1-dominant immunity in tumor immunology

To investigate the precise role of antigen-specific Th1 and Th2 cells in tumor immunity, we developed a novel adoptive tumor-immunotherapy model using OVA-specific Th1 and Th2 cells and an OVA gene-transfected tumor. This therapeutic model demonstrated that both antigen-specific Th1 and Th2 cells had strong antitumor activity in vivo with distinct mechanisms. However, immunological memory suitable for the generation of tumor-specific cytotoxic T lymphocytes was induced only when tumor-bearing mice received Th1 cell therapy, but not Th2 cell therapy. Thus it was strongly suggested that Th1-dominant immunity is critically important for the induction of antitumor cellular immunity in vivo. We also proposed that several immunomodulating protocols using interleukin (IL)-12, IL-12 gene, the natural killer T cell ligand !-galactosylceramide, or Th1 cytokine-conditioned dendritic cells might be useful strategies for the induction of Th1-dominant immunity essential for the development of tumor-specific immunotherapy.     

Importance of tumor-specific cytotoxic CD8+ T-cells in eradication of a large subcutaneous MOPC-315 tumor following low-dose melphalan therapy

We have previously demonstrated that depletion of CD8+ T-cells by the use of a monoclonal anti-Lyt-2.2 antibody abolishes the curative effectiveness of low-dose melphalan (L-phenylalanine mustard; L-PAM) therapy for BALB/c mice bearing a large (greater than or equal to 20 mm) s.c. MOPC-315 tumor and extensive metastases (Mokyr et al., Cancer Res., 49: 4597-4606, 1989). Here we show that as a consequence of low-dose L-PAM therapy, CD8+ T-cells accumulate in the s.c. tumor nodules of MOPC-315 tumor bearers. Specifically, an 80-fold increase in the number of CD8+ T-cells was seen within 5 days after the chemotherapy. Treatment of MOPC-315 tumor bearers with low-dose L-PAM in conjunction with monoclonal anti-Thy-1.2 or anti-Lyt-2.2 antibody, in contrast to treatment with monoclonal anti-L3T4 antibody, prevented the appearance of the massive CD8+ T-cell infiltrate in the s.c. tumor nodules. Fresh CD8+ T-cells derived from s.c. MOPC-315 tumor nodules that were regressing as a consequence of low-dose L-PAM therapy exhibited a potent direct lytic activity against the MOPC-315 plasmacytoma in a short-term in vitro assay. The specificity of the lytic activity exhibited by the CD8+ T-cells was illustrated not only by the inability of the CD8+ T-cells to lyse two antigenically unrelated thymomas (the WEHI 22.1 and the EL-4) and a natural killer-sensitive lymphoma (the YAC-1), but also by their relatively weak lytic activity against an antigenically related plasmacytoma (the MOPC-104E). Thus, CD8+ T-cells that infiltrate the s.c. tumor nodules of MOPC-315 tumor bearers following low-dose L-PAM therapy most likely exploit a CTL-type lytic mechanism to eradicate at least part of the large tumor burden not eliminated by the direct antitumor effects of the drug.     

The role of anti-asialo GM1 antibody-sensitive cells in the implementation of tumor-specific T cell-mediated immunity in vivo

The present study deals with the role of cells sensitive to anti-asialo GM1 antibody treatment in T cell-mediated tumor cell eradication in vivo. Rabbit anti-asialo GM1 antiserum was injected into C3H/He mice. This treatment not only resulted in almost complete abrogation of natural killer (NK) cell activity but also produced a potent inhibiting effect on the generation of activated macrophage activity induced by inoculating Propionibacterium acnes (P. acnes). Such an immunodepressed state lasted for 20 days or more after 5 consecutive injections of anti-asialo GM1 antiserum. These anti-asialo GM1 antibody-treated C3H/He mice were used as recipients in Winn assays, in which the neutralizing activity of spleen cells immunized to syngeneic X5563 tumor cells was assessed. The results demonstrated that anti-X5563 immune spleen cells depleted of asialo GM1-positive cells by the in vitro treatment with anti-asialo GM1 antibody plus complement exhibited potent anti-X5563 tumor-neutralizing activity in antibody-untreated normal recipient mice. In contrast, the X5563-immune spleen cells depleted of asialo GM1+ cells failed to produce tumor protection in asialo GM1 antiserum-treated recipient mice. When T cell-deprived B cell mice were used as recipients in Winn assays, X5563 immune spleen cells depleted of asialo GM1+ cells exhibited or failed to exhibit tumor-neutralizing activity in asialo GM1 antiserum-untreated or -treated recipient B cell mice, respectively. These results indicate that the implementation of T cell-mediated in vivo protective immunity requires the participation of anti-asialo GM1 antibody-sensitive cells, but not necessarily the host's T cells.     

Gene therapy with RNAi targeting UHRF1 driven by tumor-specific promoter inhibits tumor growth and enhances the sensitivity of chemotherapeutic drug in breast cancer in vitro and in vivo

UHRF1, also known as ICBP90 (inverted CCAAT box binding protein 90) in human, is a nuclear protein that acts as a fundamental regulator in cell proliferation and maintains DNA methylation. It is reported that UHRF1 is obviously upregulated in various human malignancies, but unchanged in differentiated tissues, suggesting that UHRF1 plays a crucial role in carcinogenesis and can be a useful anticancer drug target. In this study, we explored whether UHRF1 can be a therapeutic target for human breast carcinoma. We successfully constructed the tumor-specific shRNA expression vector driven by survivin promoter targeting UHRF1 gene. The tumor-specific RNA interference system efficiently and specifically knocked down UHRF1 expression, induced the apoptosis of tumor cells, and enhanced chemosensitivity of tumor cells to cisplatinum, but not in normal cells in vitro and in vivo. Therefore, the survivin promoter-driving shRNA expression system targeting UHRF1 may play a vital and potential role for the treatment of specificity and high efficacy in human breast carcinomas.     

自体肿瘤瘤苗对荷瘤鼠Th1、Th2型细胞因子及细胞毒作用的影响

目的研究经处理的H22肝癌细胞肿瘤瘤苗作为全细胞瘤苗对H22荷瘤小鼠体内Th1/Th2细胞比例和细胞因子的影响以及CTL的杀伤活性.方法用加重组白细胞介素2、重组粒细胞单核细胞集落刺激因子及福氏不完全佐剂制成疫苗,建立荷瘤小鼠模型,用51Cr释放法测定瘤苗免疫组、荷瘤组、正常组小鼠脾细胞对亲本H22肝癌细胞的杀伤活性;流式细胞仪检测单个核细胞中的Th1和Th2细胞,并取血检测血清中IL-10、IFN-γ水平.结果效靶比为200:1时,免疫小鼠脾细胞体外杀伤亲本H22肝癌细胞的杀伤率为38.3%,显著高于荷瘤组的13.6%,正常组的7.5%,以及对S180细胞的9.1%(P均＜0.05).瘤苗免疫组Th1细胞及Th1/Th2细胞的比值显著升高(P＜0.01),血清IFN-γ较对照组明显升高(P＜0.01);血清IL-10较对照组明显降低(P＜0.01).结论肿瘤细胞加小剂量IL-2和GM-CSF及佐剂组成的肿瘤细胞瘤苗可激发特异性细胞介导的免疫反应,改善抗肿瘤免疫反应.     

Critical impact of the kinetics of dendritic cells activation on the in vivo induction of tumor-specific T lymphocytes

Dendritic cells (DCs) need activation for the priming of antigen-specific immune responses. Recently activated DCs were described to prime in vitro strong T helper cell type 1 (Th(1)) responses, whereas at later time points, the same cells preferentially prime Th(2) cells [Langenkemp, A. et al., Nat. Immunol. 1: 311-316, 2000]. Because the immune response against cancer strongly depends on CTLs of Th(1)-like phenotype (Tc(1)), we verified here whether the kinetics of DCs activation also impacted on in vivo priming of tumor-specific CTLs. After pulsing with the CTL epitope TRP-2(180-188), bone-marrow-derived DCs, exposed to lipopolysaccharide (LPS) for 8 h (8hDC), elicited a more powerful Tc(1) response in C57BL/6 mice than did untreated DCs, or DCs exposed to LPS for 48 h (48hDC). Indeed, 8hDCs were the most potent protective and therapeutic vaccine against B16 melanoma. Despite a higher expression of MHC and costimulatory molecules by 48hDCs, 8hDCs and 48hDCs showed comparable allostimulatory and migration potential, and susceptibility to CTL-mediated apoptosis. However, 8hDCs exhibited a significantly higher interleukin (IL)-12 production potential. Release of IL-12 was necessary to induce potent Tc(1) cells, because DCs from IL-12p40(-/-) mice, irrespective of their maturation level, generated low CTL responses, comparable with 48hDCs and 0hDCs from wild-type animals. Our data are relevant for the design of DC-based vaccines.     

4-1BB在肿瘤治疗中的作用

4-lBB是肿瘤坏死因子受体(TNFR)超家族成员,具有影响T细胞扩增和存活的功能特性.通过4-1BBL或者抗4-1BB单克隆抗体干预4-1BB协同刺激途径可能对肿瘤有治疗作用.最近提出的s4-1BBL即4-1BBL可溶性形式,对恶性血液病的诊断和治疗以及判断预后有一定价值.现综述有关4-lBB在肿瘤治疗中的研究进展.     

Roles of idiotype-specific t cells in myeloma cell growth and survival: Th1 and CTL cells are tumoricidal while Th2 cells promote tumor growth

Idiotype (Id) protein, secreted by myeloma cells, is a tumor-specific antigen. Id-based immunotherapy has been explored in patients with myeloma, and results were disappointing. Although previous studies have shown that Id-specific CTLs are able to lyse myeloma cells, it is unclear whether other types of Id-specific T cells, such as type-1 T-helper (Th1) and type-2 T-helper (Th2) cells, are also able to suppress or kill myeloma cells. Using a 5T murine myeloma model, we generated T-cell clones of different subsets and examined their function in the context of myeloma cells. Id-specific CTLs specifically lysed myeloma cells via MHC class I, perforin, and Fas ligand (FasL), and Th1, but not Th2, cells lysed the myeloma cells by FasL-Fas interaction. CTL and Th1 cells also suppressed the growth and function of myeloma cells, whereas Th2 cells promoted the proliferation and enhanced the secretion of Id protein and cytokines by myeloma cells. CTL and Th1, but not Th2, cells were able to eradicate established myeloma in vivo after adoptive transfer. These results show that Id-specific CTL and Th1 are promising effector cells, whereas Th2 provide no protection and may even promote tumor progression in vivo.     

Critical role for Gab2 in transformation by BCR/ABL

The BCR/ABL oncogene causes chronic myelogenous leukemia (CML) in humans and a CML-like disease, as well as lymphoid leukemia, in mice. p210 BCR/ABL is an activated tyrosine kinase that phosphorylates itself and several cellular signaling proteins. The autophosphorylation site tyrosine 177 binds the adaptor Grb2 and helps determine the lineage and severity of BCR/ABL disease: Tyr177 mutation (BCR/ABL-Y177F) dramatically impairs myeloid leukemogenesis, while diminishing lymphoid leukemogenesis. The critical signal(s) from Tyr177 has remained unclear. We report that Tyr177 recruits the scaffolding adaptor Gab2 via a Grb2/Gab2 complex. Compared to BCR/ABL-expressing Ba/F3 cells, BCR/ABL-Y177F cells exhibit markedly reduced Gab2 tyrosine phosphorylation and association of phosphatidylinositol-3 kinase (PI3K) and Shp2 with Gab2 and BCR/ABL, and decreased PI3K/Akt and Ras/Erk activation, cell proliferation, and spontaneous migration. Remarkably, bone marrow myeloid progenitors from Gab2 (-/-) mice are resistant to transformation by BCR/ABL, whereas lymphoid transformation is diminished as a consequence of markedly increased apoptosis. BCR/ABL-evoked PI3K/Akt and Ras/Erk activation also are impaired in Gab2 (-/-) primary myeloid and lymphoid cells. Our results identify Gab2 and its associated proteins as key determinants of the lineage and severity of BCR/ABL transformation.     

Galectin-1 as a potent target for cancer therapy: role in the tumor microenvironment

The microenvironment of a tumor is a highly complex milieu, primarily characterized by immunosuppression, abnormal angiogenesis, and hypoxic regions. These features promote tumor progression and metastasis, resulting in poor prognosis and greater resistance to existing cancer therapies. Galectin-1 is a β-galactoside binding protein that is abundantly secreted by almost all types of malignant tumor cells. The expression of galectin-1 is regulated by hypoxia-inducible factor-1 (HIF-1) and it plays vital pro-tumorigenic roles within the tumor microenvironment. In particular, galectin-1 suppresses T cell-mediated cytotoxic immune responses and promotes tumor angiogenesis. However, since galectin-1 displays many different activities by binding to a number of diverse N- or O-glycan modified target proteins, it has been difficult to fully understand how galectin-1 supports tumor growth and metastasis. This review explores the importance of galectin-1 and glycan expression patterns in the tumor microenvironment and the potential effects of inhibiting galectin-1 as a therapeutic target for cancer treatment.     

Modulation of immune response against tumor cells by the in vivo administration of an autoreactive Th clone

The immunization of C3H mice with irradiated syngeneic MM48 tumor cells induced specific tumor-neutralizing cells (TNC). The TNC activity was mediated by the L3T4+, Ly-2- T cell population, and the generation of TNC coincided with the appearance of delayed-type hypersensitivity response against MM48 antigen. The administration of an auto-I-Ak reactive T helper cell clone MS202 to normal C3H mice resulted in the facilitation of growth of MM48 tumor due to the induction of T suppressor (Ts) cells. Splenic T cells from animals given this T cell clone inhibited the TNC activity of immunized mice resulting in the escape of MM48 from the TNC effect. The surface phenotype of the Ts cells was L3T4+, Ly-2-. The Ts cells induced by the clone MS202 were totally antigen-nonspecific, and were able to suppress both the effector and inductive processes of TNC. The results suggest the presence of a physiological MHC-restricted T cell circuit that regulates immune responses against the growing tumors.     

激动性CD40抗体通过调节Th1/Th2平衡增强抗肿瘤作用的机制研究

  目的  探讨激动性CD40抗体中单链抗体（CD40ScFv）和单克隆抗体（CD40mAb）抑制小鼠乳腺癌细胞生长情况及对癌组织中Th1和Th2平衡的改变,分析通过调节Th1/Th2平衡达到抗肿瘤作用的机制。  方法  将含CD40ScFv基因片段的重组质粒转化至Rosetta大肠杆菌中,诱导表达并经纯化后获得有功能的CD40ScFv蛋白,行SDS-PAGE和Western blot检测。体外培养小鼠乳腺癌4T1细胞,在Balb/C小鼠皮下成瘤建立模型,分为CD40ScFv组、CD40mAb组和生理盐水组（NS组）,分别给予CD40ScFv、CD40mAb和生理盐水,观察各组小鼠肿瘤体积的变化。取出瘤体组织,ELISA法检测肿瘤组织上清中IL-12的浓度。提取肿瘤组织浸润淋巴细胞（TILs）,应用流式细胞技术检测TILs中Th1与Th2的比例。  结果  诱导表达的CD40ScFv鉴定表达成功,分子大小约为27 KDa,蛋白纯化后经BCA测定浓度为1.12 mg/mL。CD40ScFv组与CD40mAb组肿瘤大小分别为（3.044±0.239）cm3与（2.749±0.261）cm 3,均小于NS组的（3.933±0.326）cm3,差异具有统计学意义（P <0.05）。CD40ScFv组、CD40mAb组肿瘤组织中IL-12浓度分别为（396.27±48.13）pg/mL、（457.63±58.37）pg/mL,均显著高于NS组的（79.51±14.97）pg/mL,差异具有统计学意义（P <0.05）。CD40ScFv组与CD40mAb组的Th1/Th2细胞比值分别为6.32±0.87与5.54±0.71,均显著高于NS组的1.79±0.38,差异具有统计学意义（P < 0.05）。  结论  激动性CD40抗体在体内通过调节Th1与Th2的比例,发挥抑制肿瘤生长的作用,其中通过促进DC活化后分泌IL-12是影响肿瘤微环境中Th1/Th2平衡的重要机制之一。      

A critical role for GRP78/BiP in the tumor microenvironment for neovascularization during tumor growth and metastasis

Glucose-regulated protein 78 (GRP78)/BiP is a multifunctional protein which plays a major role in endoplasmic reticulum (ER) protein processing, protein quality control, maintaining ER homeostasis, and controlling cell signaling and viability. Previously, using a transgene-induced mammary tumor model, we showed that Grp78 heterozygosity impeded cancer growth through suppression of tumor cell proliferation and promotion of apoptosis and the Grp78(+/-) mice exhibited dramatic reduction (70%) in the microvessel density (MVD) of the endogenous mammary tumors, while having no effect on the MVD of normal organs. This observation suggests that GRP78 may critically regulate the function of the host vasculature within the tumor microenvironment. In this article, we interrogated the role of GRP78 in the tumor microenvironment. In mouse tumor models in which wild-type (WT), syngeneic mammary tumor cells were injected into the host, we showed that Grp78(+/-) mice suppressed tumor growth and angiogenesis during the early phase but not during the late phase of tumor growth. Growth of metastatic lesions of WT, syngeneic melanoma cells in the Grp78(+/-) mice was potently suppressed. We created conditional heterozygous knockout of GRP78 in the host endothelial cells and showed severe reduction of tumor angiogenesis and metastatic growth, with minimal effect on normal tissue MVD. Furthermore, knockdown of GRP78 expression in immortalized human endothelial cells showed that GRP78 is a critical mediator of angiogenesis by regulating cell proliferation, survival, and migration. Our findings suggest that concomitant use of current chemotherapeutic agents and novel therapies against GRP78 may offer a powerful dual approach to arrest cancer initiation, progression, and metastasis.     

半枝莲多糖对C26荷瘤小鼠血清Th1/Th2亚群细胞因子的影响

目的观察半枝莲多糖(scutellaria barbata polysaccharides,SPS)对C26荷瘤小鼠血清Th1/Th2亚群细胞因子的影响。方法建立C26结肠癌小鼠移植瘤模型,SPS作用10天后,通过眼眶静脉采血,分离血清,ELISA法测定Th1亚群细胞因子IFN-γ、IL-2和Th2亚群细胞因子IL-4、IL-10的含量。结果 SPS 100 mg·kg~(-1)·d~(-1)、200mg·kg~(-1)·d~(-1)剂量组治疗组与模型组比较,IL-4和IL-10水平明显下降,IFN-γ和IL-2水平明显上升,差异有统计学意义。结论 SPS可以调节荷瘤小鼠Th1/Th2平衡。     

Soluble human LAG-3 molecule amplifies the in vitro generation of type 1 tumor-specific immunity

The adjuvant activities of the human lymphocyte activation gene-3 (LAG-3) molecule have been evaluated in a human setting by investigating the ability of a soluble recombinant human LAG-3 protein (hLAG-3Ig) to enhance the in vitro induction of viral- and tumor-specific CTLs. We found that soluble human LAG-3 significantly sustained the generation and expansion of influenza matrix protein Melan-A/MART-1 and survivin-specific CD8+ T lymphocytes in peripheral blood mononuclear cells (PBMC) of both cancer patients and healthy donors, showing its ability to boost CD8+ T-cell memory response or to prime naive T cells in vitro. The peptide-specific T cells generated in the presence of hLAG-3Ig were endowed with cytotoxic activity and enhanced release of type 1 cytotoxic T (Tc1) cytokines and were able to recognize tumor cells expressing their nominal antigen. Phenotype and cytokine/chemokines produced by antigen-presenting cells (APC) of PBMCs exposed in vitro for 2 days to peptide and hLAG-3Ig indicate that the LAG-3-mediated adjuvant effect may depend on a direct activation of circulating APCs. Our data revealed the activity of hLAG-3Ig in inducing tumor-associated, antigen-specific CD8+ T-cell responses in a human setting and strongly support the conclusion that this recombinant protein is a potential candidate adjuvant for cancer vaccines.     

Evaluation of oxygen diffusion distances in human breast cancer xenografts using tumor-specific in vivo data: role of various mechanisms in the development of tumor hypoxia

Oxygen diffusion distances in human breast cancer xenografts are computed considering cell line-specific in vivo data. By introducing variations into a set of experimental data characterizing a "standard" situation, the impacts of various tumor-specific or systemic mechanisms are studied. Evidence is given that radioresistance may develop at intercapillary distances above 100 micron. If intercapillary distances exceed 140 micron, hypoxia is present at the arterial end of microvessels already. Hypoxia evidenced under "standard" conditions is distinctly aggravated by reduction of microvascular blood flow, increased arterio-venous shunt perfusion, reduced red blood cell flux, elongation of tumor microvessels, anemia or arterial hypoxemia. Hypoxic tissue fractions calculated for these various conditions are found to be in accordance with the proportion of tumor tissue Po2-readings at 0-1 mmHg.