Investigation of the role of CCR2 and activin A in fibrosis development during experimental autoimmune orchitis

Abstract

EAO is a mouse model of chronic testicular inflammation and fibrosis, well reflecting pathologies seen in some forms of spermatogenic disturbances in men. This model is characterized by the destruction of testicular morphology, infiltration of the testicular interstitium by leukocytes, elevated levels of pro-inflammatory cytokines/ chemokines such as TNF, CCL2 (ligand for CCR2) and activin A, as well as loss of germ cells, all leading to fibrosis and subsequent infertility. TMs constitute the principal immune cell population in the testis and these cells play a critical role in maintaining tissue homeostasis. Increased numbers of TMs during testicular inflammation correlate with a higher incidence and severity of testicular damage. Because resident or macrophages newly recruited to the inflammatory lesions can produce a variety of pro-fibrotic factors including CCL2, TGF-β, TNF, PDGFs, MMPs or TIMPs, they are key players in fibrotic remodeling. CCL2 and CCR2 have a critical role in mediating the trafficking of monocytes, macrophages or bone marrow-derived fibroblasts to the site of injury. In the EAO model, an increase of CCL2 is observed not only in the testicular tissue but also in testicular interstitial fluid and conditioned medium from cultured TMs. Furthermore, activin A, a member of the TGF-β superfamily of cytokines, released mainly by SCs is increased in EAO testis and levels correlate with the severity of disease. Activin A stimulates resting macrophages to produce inflammatory mediators and promotes the expression of fibrosis specific genes in PTCs and NIH 3T3 fibroblasts in vitro. Based on these findings, we hypothesize that activin A in concert with CCR2 influences the development of testicular fibrosis by regulating the properties of macrophages, which in turn are an important source of pro-fibrotic factors in EAO. Therefore, the aims of this study are (1) to investigate the effect of CCR2 on the development of fibrosis during testicular inflammation, and (2) to analyze the influence of activin A on the fibrotic response in macrophages.