肿瘤免疫逃逸的主要机制之一是肿瘤细胞在缺少协同激活因子表达的同时却表达高水平的协同抑制因子从而导致免疫耐受。如何调节T细胞使它们在肿瘤微环境下抵制免疫耐受，增强抗肿瘤免疫反应.是当前肿瘤免疫领域所要解决的关键问题，也是本项目拟解决的问题。细胞周期调控因子p27蛋白阻断细胞增殖反应，是T细胞的免疫耐受过程中的重要环节。p27蛋白缺失的CD4+T细胞对CD28协同激活信号的需求降低，能够抵抗由阻断CD28信号通路引起的免疫耐受。我们最近的研究表明p27基因缺陷的CD8+T细胞在受到同种异体抗原刺激时，活化和增殖较野生型细胞显著增加。我们推测p27基因缺陷的CD8+T细胞在体内能有效地抵抗肿瘤免疫耐受环境的影响，杀伤肿瘤细胞。我们拟利用体外和动物体内实验检测p27基因缺陷对CD8+Ｔ细胞介导的抗肿瘤免疫反应，并对p27蛋白调节免疫反应的分子机制加以研究，为肿瘤免疫治疗提供新方案及理论依据。

Although immune responses directed towards various tumor antigens can be detected in rodent models and cancer patients, these anti-tumor responses are incapable of eliminating cancer in vivo. Three major mechanisms appear to be responsible for the poor anti-tumor immune responses in vivo. First, the inability of tumors to initiate, amplify and maintain CTL responses because they lack costimulatory molecules, thereby inducing T cell anergy instead of priming in tumor infiltrating T lymphocytes. Second, the expression of PD-L1 by tumor cells via which they initiate PD-1-mediated negative signals to tumor infiltrating T lymphocytes, leading to their inactivation. Third, the production of soluble factors (predominantly TGF-?) by tumor cells inhibits T cell activation and expansion. p27 is a critical inhibitor of cell cycle progression. Recently, we observed that p27 deficient T cells are resistant to tolerance induction by blockade of costimulation. Consistently, p27 deficient recipients reject cardiac allografts under conditions that induce long-term allograft survival in wild-type recipients. We have also observed that p27 deficient CTL express lower levels of PD-1 than their wild-type counterparts. Finally, compared to their wild-type counterparts, p27 deficient CD8+ cells are less susceptible to the inhibitory effects of TGF-? and highly sensitive to IL-2 receptor mediated signals. These properties of p27 deficient lymphocytes suggest that elimination of p27 may overcome three major mechanisms via which tumor cells escape immune surveillance in vivo. In this proposal we will explore the hypothesis that p27 deficient CD8+ T cells may be capable of mounting effective anti-tumor responses and potentially suppress tumor growth. We will undertake the following specific aims to: 1) Examine whether p27 deficient CD8+ T cells can differentiate into efficient tumor-specific CTL under tolerogenic conditions. To address this question we shall use OT-I TCR transgenic T cells (which express a TCR specific for OVA257-264 peptide), OT-I /p27 deficient T cells (OT-I/p27?), and EG7 tumor, a syngeneic thymoma that expresses Ovalbumin. We shall examine: 1) CD8+ T cell proliferation and production of IL-2, IFN-? and TNF??in response to tumor cell stimulation; 2) generation of functional CTL, as determined by expression of perforin and granzyme B and assessment of effector function by tumor-specific killing assay in vitro. 2) Examine whether p27 deficient CTL can efficiently suppress tumor growth. We shall use the same experimental system as in Aim 1. We will examine: 1) whether adoptive transfer of OT-I /p27 deficient CD8+ T cells into tumor-bearing mice prevents the growth of tumor, and 2) the properties of adoptive transferred OT-I cells in tumor -bearing host. 3) Examine whether knockdown of p27 in OT-I cells from tumor-bearing mice enhances their cytolytic activity against EG7 tumor in vitro.