An acidic microenvironment increases NK cell killing of Cryptococcus neoformans and Cryptococcus gattii by enhancing perforin degranulation.
An acidic microenvironment increases NK cell killing of Cryptococcus neoformans and Cryptococcus gattii by enhancing perforin degranulation.
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DOI：
10.1371/journal.ppat.1003439
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发表时间：
2013
期刊：
影响因子：
6.7
通讯作者：
Mody CH
中科院分区：
文献类型：
--
作者：
Islam A;Li SS;Oykhman P;Timm-McCann M;Huston SM;Stack D;Xiang RF;Kelly MM;Mody CH
加蒂氏隐球菌和新生隐球菌是被包裹的酵母菌，可以产生固体肿块或隐球菌瘤。与恶性肿瘤类似，隐球菌体内深处的微环境是酸性的，这对宿主防御提出了独特的挑战。与恶性细胞类似，NK细胞可以杀死隐球菌。因此，就像在肿瘤防御中一样，NK细胞必须在从生理pH到隐球菌瘤中心低于6的梯度上杀死酵母细胞。由于酸性pH抑制NK细胞的抗肿瘤活性，我们试图确定NK细胞的抗隐球菌活性是否也有类似的降低。令人惊讶的是，我们发现原代人类NK细胞和人类NK细胞系YT都在酸性条件下保存了甚至增强了对隐球菌的杀灭作用，而不是生理pH。对增强杀伤机制的研究表明，酸性pH不会增加效靶比、细胞与隐球菌的结合或效应细胞中活性穿孔素的含量。相比之下，在酸性pH条件下，穿孔素的脱颗粒程度更大，在脱颗粒增加之前，ERK1/2的磷酸化增强，这是杀戮所必需的。此外，使用复制缺陷的隐球菌ras1基因敲除菌株，在更快的生物复制过程中会增加脱颗粒。最后，在致命性感染的隐球菌瘤中，发现NK细胞与加蒂卡氏菌密切相关。这些结果表明，NK细胞在低pH值和微生物快速复制时具有放大的信号、脱颗粒和更大的杀伤力，这将有助于在酸性微环境下维持杀菌宿主防御。保护免受感染的免疫反应必须发生在各种独特的和可能有害的环境中。在这些环境中，酸中毒会对保护性反应产生深远的影响。低pH值可发生在局灶性肿瘤样感染中，例如由真菌病原体隐球菌产生的隐球菌瘤。同样，局灶性恶性肿瘤也会出现低pH值。因此，NK细胞对隐球菌属和恶性细胞均有杀伤作用，是机体防御的重要机制。因此，我们询问低pH是否也会影响NK细胞对隐球菌的杀伤作用。令人惊讶的是，尽管NK细胞对肿瘤的杀伤力减弱，但在低pH值下，NK细胞对隐球菌的杀伤力增强。其机制涉及细胞内信号转导的增强，导致穿孔素脱颗粒增强。这导致我们在一种致命的脑部感染和肺部的持续性隐球菌病中检查了NK细胞。我们发现，在隐球菌瘤中，NK细胞与隐球菌结合，但穿孔素减少。这些研究表明，在低pH条件下NK细胞的细胞毒作用不一定会受到损害，而在低pH条件下加强信号转导和脱颗粒可能有助于增强宿主的防御能力。
Cryptococcus gattii and Cryptococcus neoformans are encapsulated yeasts that can produce a solid tumor-like mass or cryptococcoma. Analogous to malignant tumors, the microenvironment deep within a cryptococcoma is acidic, which presents unique challenges to host defense. Analogous to malignant cells, NK cells kill Cryptococcus. Thus, as in tumor defense, NK cells must kill yeast cells across a gradient from physiologic pH to less than 6 in the center of the cryptococcoma. As acidic pH inhibits anti-tumor activities of NK cells, we sought to determine if there was a similar reduction in the anticryptococcal activity of NK cells. Surprisingly, we found that both primary human NK cells and the human NK cell line, YT, have preserved or even enhanced killing of Cryptococcus in acidic, compared to physiological, pH. Studies to explore the mechanism of enhanced killing revealed that acidic pH does not increase the effector to target ratio, binding of cytolytic cells to Cryptococcus, or the active perforin content in effector cells. By contrast, perforin degranulation was greater at acidic pH, and increased degranulation was preceded by enhanced ERK1/2 phosphorylation, which is essential for killing. Moreover, using a replication defective ras1 knockout strain of Cryptococcus increased degranulation occurred during more rapid replication of the organisms. Finally, NK cells were found intimately associated with C. gattii within the cryptococcoma of a fatal infection. These results suggest that NK cells have amplified signaling, degranulation, and greater killing at low pH and when the organisms are replicating quickly, which would help maintain microbicidal host defense despite an acidic microenvironment. Immune responses that protect from infection must occur in a variety of unique and potentially hostile environments. Within these environments, acidosis causes profound affects on protective responses. Low pH can occur in focal tumor-like infections, such as in a cryptococcoma produced by the fungal pathogen Cryptococcus. Similarly, low pH occurs in focal malignant tumors. It follows that Cryptococcus and malignant cells can both be killed by NK cells, which provide an important mechanism of host defense. Thus, we asked whether low pH, which impairs tumor killing, might also affect NK cell killing of Cryptococcus. Surprisingly, despite impaired tumor killing, NK cells possess enhanced killing of Cryptococcus at low pH. The mechanism involved a gain in intracellular signal transduction that led to enhanced perforin degranulation. This led us to examine NK cells in persistent cryptococcoma of a fatal brain infection and lung. We found that NK cells associate with Cryptococcus within the cryptococcoma, but perforin is reduced. These studies suggest NK cell cytotoxicity need not be impaired at low pH, and that enhanced signal transduction and degranulation at low pH might be used to enhance host defense.
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