CHANGES IN HISTOLOGY AND FIBROGENIC CYTOKINES IN IRRADIATED COLORECTUM OF 2 MURINE STRAINS

Purpose: A strain difference in the development of radiation-induced f ibrosis of the colorectum was recently observed. C57B1/6 mice develope d colorectal obstruction with significantly higher incidence compared to C3Hf/Kam mice after partial volume irradiation with 30 Gy. Previous reports have demonstrated differences in cytokine mRNA levels in fibr osis-prone and -resistant mice after lung irradiation. The aims of thi s study are to determine if there are strain differences in: I) the hi stology of the lesion, 2) mRNA levels for transforming growth factor b eta (TGF beta) isoforms and tumor necrosis factor alpha (TNF alpha), a nd 3) immunohistochemical staining patterns using antibodies against t he TGF beta isoforms and latency-associated peptide (LAP). Methods and Materials: The colorectum of male C3Hf/Kam (C3H) and C57BI/6 (B6) mic e were irradiated using a dose/length combination (30 Gy to 13.7 mm) t hat resulted in 10 or 100% incidence of obstruction by 6 months in eac h strain, respectively. Colorectal tissue was removed from 6 hours to 120 days after irradiation as well as from obstructed mice and prepare d for histology, RNase protection assay, and immunofluorescence. Resul ts: Distinct differences in the histological phenotype for the two str ains were observed at times preceding obstruction. Samples from B6 mic e showed increased hyperplastic crypts, colitis cystica profunda, and fibrosis within the lamina propria, compared to identically treated C3 H mice. Fibrosis in the lamina propria of B6 mice appeared early, begi nning at 75 days after irradiation, and was progressive, whereas fibro sis in C3H mice appeared simultaneous with obstruction and may have be en a reaction to ulceration. No consistent strain difference in mRNA l evels for TGF beta 1, 2, 3 or TNF alpha were observed, although mRNA l evels of TGF beta 1 and TNF alpha were significantly elevated in both strains relative to nonirradiated controls. Immunofluorescent staining for TGF beta 1, 3 and LAP was observed in hyperplastic crypts and col itis cystica profunda adjacent to regions of fibrosis, histological ch anges that were present predominately in the B6 strain. Conclusions: T he response of the colorectum to irradiation involves changes in the e xpression of several different cytokines. However, the lack of a consi stent strain difference in TGF beta 1, 2, 3 and TNF alpha mRNA levels, despite strain differences in both the incidence of colorectal obstru ction and histological features preceding obstruction, suggests that m RNA changes in these fibrogenic cytokines are not the critical determi nant of the strain difference and are not related to the process of ra diation-induced colorectal fibrosis in these mouse strains. Strain-dep endent differences were observed in the localization of active TGF bet a, but these differences were related to the histological changes spec ifically found in the irradiated colon of the B6 strain. (C) 1998 Else vier Science Inc.