PDE8 Regulates Rapid Teff Cell Adhesion and Proliferation Independent of ICER

BackgroundAbolishing the inhibitory signal of intracellular cAMP by phosphodiesterases (PDEs) is a prerequisite for effector T (Teff) cell function. While PDE4 plays a prominent role, its control of cAMP levels in Teff cells is not exclusive. T cell activation has been shown to induce PDE8, a PDE isoform with 40- to 100-fold greater affinity for cAMP than PDE4. Thus, we postulated that PDE8 is an important regulator of Teff cell functions.
Methodology/Principal FindingsWe found that Teff cells express PDE8 in vivo. Inhibition of PDE8 by the PDE inhibitor dipyridamole (DP) activates cAMP signaling and suppresses two major integrins involved in Teff cell adhesion. Accordingly, DP as well as the novel PDE8-selective inhibitor PF-4957325-00 suppress firm attachment of Teff cells to endothelial cells. Analysis of downstream signaling shows that DP suppresses proliferation and cytokine expression of Teff cells from Crem−/− mice lacking the inducible cAMP early repressor (ICER). Importantly, endothelial cells also express PDE8. DP treatment decreases vascular adhesion molecule and chemokine expression, while upregulating the tight junction molecule claudin-5. In vivo, DP reduces CXCL12 gene expression as determined by in situ probing of the mouse microvasculature by cell-selective laser-capture microdissection.
Conclusion/SignificanceCollectively, our data identify PDE8 as a novel target for suppression of Teff cell functions, including adhesion to endothelial cells.

背景 磷酸二酯酶（phosphodiesterases, PDEs）解除细胞内环腺苷酸（cyclic adenosine monophosphate, cAMP）的抑制性信号，是效应T细胞（effector T cell, Teff）发挥功能的必要前提。尽管磷酸二酯酶4（PDE4）发挥主要调控作用，但其对Teff细胞内cAMP水平的调控并非唯一途径。已有研究证实，T细胞活化可诱导PDE8的表达——该PDE亚型对cAMP的亲和力是PDE4的40至100倍。据此，我们提出假说：PDE8是Teff细胞功能的重要调控因子。
研究方法与主要结果 我们发现，Teff细胞在体内可表达PDE8。采用PDE抑制剂双嘧达莫（dipyridamole, DP）抑制PDE8后，可激活cAMP信号通路，并抑制两种参与Teff细胞黏附的核心整合素。相应地，DP与新型PDE8选择性抑制剂PF-4957325-00均可抑制Teff细胞与内皮细胞的牢固黏附。下游信号通路分析显示，DP可抑制缺乏可诱导性cAMP早期阻遏物（inducible cAMP early repressor, ICER）的Crem基因敲除（Crem−/−）小鼠来源Teff细胞的增殖与细胞因子表达。值得注意的是，内皮细胞同样表达PDE8。DP处理可降低血管黏附分子与趋化因子的表达水平，同时上调紧密连接分子闭合蛋白-5（claudin-5）的表达。在体内实验中，通过细胞选择性激光捕获显微切割技术对小鼠微血管进行原位检测，结果显示DP可降低CXCL12的基因表达水平。
结论与意义 综上，本研究数据证实PDE8是抑制Teff细胞功能（包括其与内皮细胞的黏附）的新型治疗靶点。