Identification of Nitrated Immunoglobulin Variable Regions in the HIV-Infected Human Brain: Implications in HIV Infection and Immune Response

HIV can infiltrate the brain and lead to HIV-associated neurocognitive
disorders (HAND). The pathophysiology of HAND is poorly understood,
and there are no diagnostic biomarkers for it. Previously, an increase
in inducible nitric oxide synthase levels and protein tyrosine nitration
in the brain were found to correlate with the severity of HAND., In this study, we analyzed human brains from individuals who had
HIV infection without encephalitis and with encephalitis/HAND and
compared them to the brains of healthy individuals. We identified
the nitrated proteins and determined the sites of modification using
affinity enrichment followed by high-resolution and high-mass-accuracy
nanoLC–MS/MS. We found that nitrated proteins were predominantly
present in the HIV-infected individuals with encephalitis, and, interestingly,
the modifications were predominantly located on immunoglobulin variable
regions. Our molecular model indicated potential interactions with
HIV envelope proteins and changes on the heavy and light chain interface
upon the nitration and nitrohydroxylation of these residues. Therefore,
our findings suggest a role for these modifications in the immune
response, which may have implications in disease pathogenesis.

HIV可侵入大脑并引发HIV相关神经认知障碍（HIV-associated neurocognitive disorders, HAND）。目前HAND的病理生理机制尚未阐明，且尚无其诊断用生物标志物。既往研究发现，大脑中诱导型一氧化氮合酶（inducible nitric oxide synthase）水平升高与蛋白质酪氨酸硝化现象，均与HAND的严重程度相关。
本研究对HIV感染但未合并脑炎、以及合并脑炎/HAND的感染者的脑组织样本进行分析，并以健康个体的脑组织作为对照。通过亲和富集结合高分辨率、高质量精度的纳流液相色谱-串联质谱（nanoLC–MS/MS）技术，我们鉴定了发生硝化修饰的蛋白质，并确定了其修饰位点。研究发现，硝化蛋白质主要存在于合并脑炎的HIV感染者样本中；值得注意的是，此类修饰位点主要定位于免疫球蛋白可变区。分子模型分析表明，这些残基发生硝化及硝基羟基化修饰后，可能与HIV包膜蛋白产生相互作用，并改变免疫球蛋白重链与轻链的界面结构。因此，本研究结果提示这类修饰在免疫应答中发挥作用，或对疾病的发病机制具有潜在影响。