间皮起源于中胚层， 由一层特殊的鳞状间皮细胞组成，排列在身体的浆液腔和内部器官的表面，包括胸膜腔，心包腔，腹膜腔以及心脏，肺脏和肝脏等。间皮细胞不仅为内脏器官提供了一个光滑而不粘连的保护性表面，还具有参与炎症应答、抗原呈递等免疫调节相关的功能。
肺脏是一个复杂的器官，其内部由气道、血管系统和间质组成，表面被胸膜间皮所包裹， 然而， 胸膜间皮细胞在肺疾病中的功能研究被严重忽视。胸膜异常增厚经常在肺移植后慢性移植肺功能障碍（chronic lung allograft dysfunction， CLAD） 患者中发生，而且与更快的疾病进展相关。 CLAD 中的胸膜异常很可能伴随着胸膜间皮细胞的细胞行为改变， 但间皮细胞在胸膜增厚中的细胞学行为及其调控机制目前并不清楚，我们借助 CLAD 患者肺组织样本和主要组织相容性复合物（ majorhistocompatibility complex， MHC） 半匹配的原位单肺移植诱导的 CLAD 小鼠模型来研究这一问题。
同种异体小鼠肺移植后， 移植的异体肺显示出显著的胸膜增厚，炎症浸润和气道病变。 免疫荧光染色显示，相比于对照肺组织， 移植后 35 天的异体肺的间皮细胞数量显著增多，聚集在增厚的胸膜区域，并随着 CLAD 的进展向肺实质区域侵袭迁移。基因表达分析表明这些间皮细胞发生了细胞增殖、 间皮细胞-间充质转化，同时高表达免疫相关基因，尤其上调二型免疫相关细胞因子白介素（interlukin， IL）-33 的表达。为了研究这些间皮细胞所表达的 IL-33 功能， 我们首先通过谱系追踪发现了这些间皮细胞来自受体小鼠而非供体小鼠。小鼠遗传学实验表明， 受体间皮细胞 Il33 条件性敲除能显著减轻炎症细胞浸润，胸膜增厚以及气道病理改变。这些结果表明受体间皮细胞来源的 IL-33 贡献于 CLAD 的进展。
单细胞测序分析发现 IL-33 的受体编码基因 Il1rl1 特异地表达在嗜碱性粒细胞上， 且嗜碱性粒细胞在 CLAD 肺组织中大量浸润。为了验证 IL-33-嗜碱性粒细胞的上下游关系，我们使用 Mcpt8-Cre， Il1rl1F/F 小鼠特异性敲除嗜碱性粒细胞的 IL-33受体，发现嗜碱性粒细胞的 Il1rl1（interleukin 1 receptor like 1）敲除导致其浸润减少，同时显著减轻 CLAD 的病理学改变。
然而， 间皮细胞的条件性 Il33 敲除也能减少胸膜增厚的表型和间皮细胞数量，这表明嗜碱性粒细胞可能反过来促进间皮细胞的激活。 我们发现异体肺间皮细胞存在特异性的细胞外信号调节激酶（receptor tyrosine kinases 1/2, ERK1/2） 磷酸化。 而ERK1/2 抑制剂 U0126 的体内处理显著减少了胸膜增厚以及间皮细胞数量， 也减轻了免疫浸润和气道病变，表明 ERK1/2 磷酸化对间皮细胞的活化是重要的。 同时，我们进一步探究 ERK1/2 磷酸化的上游信号，系统分析 ERK1/2 经典上游受体酪氨酸激酶后发现，胸膜增厚过程中，间皮细胞的 Met（MET Proto-Oncogene, ReceptorTyrosine Kinase） 表达上调，并存在 Met 的磷酸化；而 Met 的配体，肝细胞生长因子（hepatocyte growth factor, HGF） 特异性高表达在嗜碱性粒细胞上，提示嗜碱性粒细胞来源的 HGF 通过 HGF-Met 信号通路反过来激活间皮细胞。为了验证 HGF和 pERK1/2 促进间皮细胞增殖的功能，我们通过原代间皮细胞体外培养，发现 HGF可以促进间皮细胞的增殖，而 ERK1/2 抑制剂 U0126 可以抑制 HGF 引起的细胞增殖。嗜碱性粒细胞的清除也减轻了间皮增厚和间皮细胞的增多，并显著抑制了 CLAD的进展。 这表明嗜碱性粒细胞与间皮细胞之间存在反馈环路。
综上，我们发现在同种异体移植后的慢性损伤过程中， 异体肺原有的间皮细胞受到损伤并发生细胞死亡，而受体来源的间皮细胞迁移到异体肺表面， 并高表达IL-33， 而间皮细胞来源的 IL-33 激活嗜碱性粒细胞，激活的嗜碱性粒细胞产生大量炎症因子， 促进炎症浸润和气道病变；同时，嗜碱性粒细胞通过 HGF 促进高表达IL33 的间皮细胞的增殖。 间皮细胞与嗜碱性粒细胞形成一个环路， 共同促进 CLAD的快速进展。

Mesothelium originates from the mesoderm and is composed of a special layer of squamous mesothelial cells, lining the serous cavities and the surfaces of internal organs, including the pleural cavity, pericardial cavity, peritoneum, as well as organs like the heart, lungs, and liver. Mesothelial cells not only provide a smooth, non-adherent protective surface for internal organs but also participate in immune regulation functions, such as inflammatory response and antigen presentation.
The lung is a complex organ composed of airways, vasculature, and stroma, covered by pleural mesothelium. However, the role of pleural mesothelial cells in lung disease has been largely overlooked. Pleural thickening frequently occurs in patients with chronic lung allograft dysfunction （ CLAD） and is associated with faster disease progression. Abnormal pleural conditions in CLAD are likely accompanied by altered cellular behavior of pleural mesothelial cells, but the cellular behaviors and regulatory mechanisms of mesothelial cells in pleural thickening remain unclear. To investigate this, we utilized lung tissue samples from CLAD patients and a murine model of CLAD induced by MHC-mismatched orthotopic single lung transplantation. 

Following allogeneic mouse lung transplantation, the allograft showed significant pleural thickening, inflammatory infiltration, and airway lesions. Immunofluorescence staining revealed that compared to control lung tissue, the number of mesothelial cells in the allograft increased significantly 35 days post-transplantation, aggregating in thickened pleural areas and migrating into the lung parenchyma as CLAD progressed. Gene expression analysis indicated that these mesothelial cells underwent cell proliferation and mesothelial-to-mesenchymal transition （ MMT） , with high expression of immune-related genes, particularly upregulating the type II immunity-related cytokine IL-33. To study the function of IL-33 expressed by these mesothelial cells, we first used lineage tracing to confirm that these cells originated from the recipient mice, not the donor. Genetic experiments in mice showed that conditional deletion of Il33 in recipient mesothelial cells significantly reduced inflammatory cell infiltration, pleural thickening, and airway pathology. These findings suggest that IL-33 derived from recipient mesothelial cells contributes to the progression of CLAD.

Single-cell sequencing analysis revealed that the gene encoding the IL-33 receptor, Il1rl1, was specifically expressed in basophils, which were heavily infiltrated in CLAD lung tissue. To validate the upstream-downstream relationship between IL-33 and basophils, we used Mcpt8-Cre, ST2F/F mice to specifically delete the IL-33 receptor in basophils. Basophil-specific deletion of ST2 led to reduced basophil infiltration and significantly mitigated CLAD-related pathological changes.

Since conditional deletion of Il33 in mesothelial cells reduced pleural thickening and the number of mesothelial cells, this suggests that basophils may in turn promote mesothelial cell activation. We found specific phosphorylation of ERK1/2 in allograft mesothelial cells. Treatment with the ERK1/2 inhibitor U0126 in vivo significantly reduced pleural thickening, the number of mesothelial cells, immune infiltration, and airway lesions, indicating that ERK1/2 phosphorylation is crucial for mesothelial cell activation. Meanwhile, we further investigated the upstream signals of ERK1/2 phosphorylation. Systematic analysis of the classical upstream of ERK1/2, receptor tyrosine kinases （ RTKs） , revealed that during pleural thickening, mesothelial cells exhibited upregulated expression of Met, along with Met phosphorylation. The ligand for Met, HGF, was specifically expressed by basophils, suggesting that HGF from basophils activates mesothelial cells through the HGF-Met signaling pathway. To verify the role of HGF and pERK1/2 in promoting mesothelial cell proliferation, we conducted in vitro primary mesothelial cell culture, showing that HGF stimulated mesothelial cell proliferation, while ERK1/2 inhibition with U0126 blocked HGF-induced proliferation. Basophil depletion also alleviated pleural thickening and mesothelial cell  proliferation, significantly inhibiting the progression of CLAD. This indicates a feedback loop between basophils and mesothelial cells. 

In summary, during chronic injury following allogeneic transplantation, the original mesothelial cells of the allograft are damaged and undergo cell death, while recipient-derived mesothelial cells migrate to the surface of the allograft and highly express IL-33. IL-33 from mesothelial cells activates basophils, which release large amounts of inflammatory factors, promoting inflammation and airway lesions.
Concurrently, basophils stimulate the proliferation of IL-33-expressing mesothelial cells through the HGF-Met signaling pathway. A feedback loop between mesothelial cells and basophils is formed, driving the rapid progression of CLAD.