Role of Formylpeptide Receptors in Host Defense

FPRL1和mFPR2是人和小鼠髓系细胞上的受体，介导细胞对病原体和宿主衍生多肽的趋化作用，包括与阿尔茨海默病(AD)相关的血清淀粉样蛋白A(SAA)和淀粉样β蛋白多肽(Aβ)。我们发现，当与被认为是脑巨噬细胞的混合胶质细胞上的FPRL1或mFPR2结合时，淀粉样β42和FPRL1复合体内化到细胞的胞浆中，长期暴露于淀粉样β42/FPRL1复合体导致淀粉样β42/FPRL1复合体滞留在细胞中，随后形成刚果红阳性纤维。相反，巨噬细胞/小胶质细胞短暂暴露于Abeta42肽也会导致Abeta42肽的摄取，但没有形成纤维聚集，这表明Abeta42肽的负荷较低。在从新生小鼠分离的小胶质细胞中，用各种促炎剂处理，如Toll样受体(TLRs)的配体、TNFpha、IFNGamma和CD40，会增加mFPR2的表达，mFPR2是小鼠的人类FPRL1的对应物。激活的小鼠小胶质细胞对淀粉样β42多肽表现出强烈的趋化反应，并通过受体mFPR2摄取多肽。我们的观察表明，FPRL1和它的小鼠对应物可能在AD中作为中枢神经系统中过量产生的Abeta42多肽的传感器。Abeta42-FPRL1(MFPR2)内化导致促炎介质的产生和Abeta42肽的加工，这可能决定AD病理的进展速度。为了更准确地评估FPRL1(MFPR2)在先天性宿主防御、炎症和AD发病机制中的作用，我们产生了一个缺失mFPR2的小鼠品系。我们正在进行的研究表明，在AD的小鼠模型中，mFPR2的缺失减少了脑中激活的小胶质细胞的数量，这与ABATA42肽沉积的水平增加和分布更分散有关。因此，FPRL1(MFPR2)似乎在宿主防御中发挥重要作用，有利于Abeta42肽在吞噬细胞中的积累，从而促进AD的清除和减轻损伤。MFPR2-/-小鼠的产生也为我们提供了一个独特的工具来研究这种受体在其他促炎和免疫疾病中的作用以及在癌症的发展中的作用。我们发现，在卵蛋白(OVA)诱导的肺部疾病炎症和免疫反应模型中，与野生型小鼠相比，mFPR2-/-在肺组织和支气管腔中的白细胞浸润明显减少，与对OVA的抗体反应降低有关。因此，我们的研究表明mFPR2在对外来抗原的炎症和免疫反应中起着关键作用。进一步的研究正在进行中，以确定mFPR2在炎症性肠病诱导的结肠癌以及在小鼠肺癌和黑色素瘤转移中的作用。

FPRL1 and mFPR2 are receptors on human and mouse myeloid cells that mediate cell chemotaxis to a pathogen and host derived peptides, including serum amyloid A (SAA) and amyloid beta peptides associated with Alzheimer's disease (AD). We have found that upon binding to FPRL1 or mFPR2 on migcroglial cells, considered as brain macrophages, Amyloid beta42 and FPRL1 complexes were internalized into the cytoplasmic compartment of the cells and prolonged exposure to Amyloid(A) beta42 resulted in the retention of Amyloid beta42/FPRL1 complexes in the cells, followed by formation of Congo-red positive fibrils. In contrast, brief exposure of macrophages/microglial cells to Abeta42 peptides also resulted in Abeta42 peptide ingestion, but without formation of fibrillary aggregation, suggesting a lower burden of Abeta42 peptides could be degraded. In microglial cells isolated from new born mice, treatment with a variety of proinflammatory agents such as the ligands for the Toll like receptors (TLRs), TNFalpha, IFNgamma and CD40 increases the expression of mFPR2, the mouse counterpart of human FPRL1. Activated mouse microglial cells exhibited potent chemotactic responses to Amyloid beta42 peptides and ingested the peptides through the receptor mFPR2. Our observations suggest that FPRL1 and its mouse counterpart may act as a sensor in the CNS for over produced Abeta42 peptides seen in AD. Abeta42-FPRL1(mFPR2) internalization results in production of proinflammatory mediators and the processing of Abeta42 peptides, which may determine the rate of the progression of AD pathology. To more precisely evaluate the role of FPRL1 (mFPR2) in innate host defense, inflammation and in the pathogenesis of AD, we have generated a mouse strain depleted of mFPR2. Our ongoing studies have revealed that in a mouse model of AD, depletion of mFPR2 reduced the number of activated microglial cells in the brain in association with increased level and more diffused distribution of Abata42 peptide deposition. Thus, FPRL1 (mFPR2) appears to play an important role in host defense favoring the accumulation of Abeta42 peptides in phagocytic microglial cells thus facilitating clearance and reducing damage in AD. The generation of mFPR2-/- mice also provides us with a unique tool to study the role of this receptor in other proinflammtory and immune diseases as well as in the development of cancer. We found that in an ovalbumin (OVA)-induced inflammatory and immune response model of lung disease, as compared with wild type mice, mFPR2-/- showed markedly reduced leukocyte infiltration in the lung tissue and in the bronchial lumen, in association with reduced antibody responses to OVA. Thus our studies suggest a key role of mFPR2 in inflammatory and immune responses to foreign antigen. Further studies are underway to determine the role of mFPR2 in inflammatory bowel disease-induced colon cancer and in the development of metastasis in mouse lung cancer and melanoma.