Understanding Immunomodulation by Candida albicans

描述(申请人提供)：白色念珠菌是哺乳动物共生菌群中普遍存在的一部分，也是人类最常见的系统性真菌病原体。系统性疾病是第四大常见的医院感染，由于感染的严重性和目前诊断和治疗的不足，约有40%的死亡率与系统性疾病有关。系统性或播散性念珠菌病主要发生在先天免疫系统因疾病、化疗或药物干预(手术或植入设备，如导管)而受到损害的患者。因此，作为共生菌的念珠菌和作为病原体的念珠菌之间有一条细微的界限，我们的长期目标是了解这种平衡是如何维持或破坏的，以促进一种或另一种状态。我们的前提是，加强免疫系统或削弱真菌，即使是轻微的，可能会使这种平衡有利于患者，因此我们研究了白色念珠菌与天然免疫系统细胞之间的相互作用。从这些研究中，我们有证据表明白色念珠菌分泌一种免疫调节化合物(S)，该化合物可以抑制巨噬细胞释放一氧化氮(NO)，一氧化氮是一种关键的抗菌和免疫调节化合物。我们已经开始表征这种抑制剂，并发现它是小的，亲水的，热稳定的，不是以碳水化合物为基础的；这项提议的一个目的是鉴定这种化合物。在这样做的过程中，我们得到了一系列基因组实验的帮助，这些实验定义了白色念珠菌对巨噬细胞吞噬作用的广泛而复杂的转录反应。最令人惊讶的发现之一是诱导了整个精氨酸生物合成途径。没有其他的核苷酸或氨基酸途径被诱导，这使得这是一种特殊和独特的反应。精氨酸除了是一种必需的氨基酸外，也是诱导型一氧化氮合酶(NOS2或iNOS)产生NO的底物。精氨酸类似物可抑制诱导型一氧化氮合酶；其中一种类似物已被证明可大大降低中性粒细胞的抗念珠菌活性。这些精氨酸类似物符合我们对白色念珠菌来源的抑制物的初步化学特征。因此，我们的中心假设是白色念珠菌已经选择了精氨酸生物合成途径来产生一种与精氨酸相关的iNOS抑制剂，从而促进这种生物的生存和发病。这里提出的实验将检验这一假设，并确定抑制化合物。严重的真菌感染，主要由假丝酵母菌引起，越来越常见和严重。这些疾病主要影响已经因其他治疗或疾病而虚弱的患者，这表明加强这些患者的免疫系统将有助于对抗这些感染。我们在这里提供的数据表明，真菌本身可能正在积极损害免疫系统，并建议表征这一过程是如何发生的，希望最终能抵消这一能力。

DESCRIPTION (provided by applicant): Candida albicans is both a ubiquitous part of the mammalian commensal flora and the most common systemic fungal pathogen of humans. Systemic disease is the fourth most common nosocomial infection and is associated with a ~40% mortality, due both to the severity of the infection and current inadequacies in diagnosis and treatment. Systemic, or disseminated, candidiasis mostly develops in patients whose innate immune system has been compromised by disease, chemotherapy, or medical intervention (surgery or implanted devices such as catheters). Thus, there is a fine line between Candida as a commensal and Candida as a pathogen and our long-term goal is to understand how this balance is maintained or disrupted to promote one state or the other. It is our premise that strengthening the immune system or weakening the fungus, even slightly, may tip this balance in favor of the patient, thus we have studied the interaction between C. albicans and cells of the innate immune system. From these studies, we have evidence that C. albicans secretes an immunomodulatory compound(s) that inhibits the release of nitric oxide (NO), a key antimicrobial and immunomodulatory compound, from macrophages. We have begun to characterize this inhibitor and have found that it is small, hydrophilic, heat-stable, and is not carbohydrate-based; one aim of this proposal is to identify this compound. In doing so, we have been aided by a series of genomic experiments that have defined the extensive and complex transcriptional response of C. albicans to phagocytosis by macrophages. One of the most surprising findings is an induction of the arginine biosynthesis pathway in its entirety. No other nucleotide or amino acid pathway is induced, making this a specific and unique response. In addition to being an essential amino acid, arginine is also the substrate for production of NO by the inducible Nitric Oxide Synthase (NOS2 or iNOS). Analogs of arginine inhibit iNOS; one such analogue has been shown to greatly reduce the anti-Candida activity of neutrophils. These arginine analogs fit our preliminary chemical characterization of the C. albicans-derived inhibitor. Thus, our central hypothesis is that C. albicans has co- opted the arginine biosynthesis pathway to produce an iNOS inhibitor, related to arginine, that promotes survival and pathogenesis of this organism. The experiments proposed here will test this hypothesis and identify the inhibitory compound. Serious fungal infections, caused mostly by Candida species, are increasingly common and severe. These affect mostly patients already debilitated by other medical treatments or illnesses and suggest that strengthening these patients' immune system would help fight these infections. We present data here that indicates that the fungus itself may be actively impairing the immune system and propose to characterize how this process occurs with the hope of eventually counteracting this ability.