Targeting of chemical mutagens to differentiating B‐lymphocytes in vivo: Detection by direct DNA labeling and sister chromatid exchange induction

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Abstract
In vivo systems for analyzing mutagen interactions with a specific differentiating cell population are rare. Taking advantage of the unique anatomical features of the bursa of Fabricius in the chicken, we explored the possibility of targeting chemical mutagens to a defined differentiating cell population in the animal, namely, the B‐lymphocytes series. Such cells are known to be the targets for the oncogene‐activating avian leukosis virus. Targeting of chemicals to cells of the bursa was demonstrated by application of the DNA‐specific fluorochrome 4′‐6‐diamidino‐2‐phenylindole (DAPI) to the anal lips of neonatal chicks. Bright nuclear fluorescence of cells in the bursa was demonstrated to occur within minutes after the application of 500 m̈l of DAPI. DAPI labeling of nuclei was detected up to several days after a single application. No nuclear labeling was exhibited in cells of neighboring tissues. Methyl methanesulfonate (MMS) (10 m̈l) was applied to the anal lips of day‐old chicks to study dose‐response kinetics for mutagen targeting to DNA of dividing B‐lymphocytes in the bursa. Since the mitotic index was found to be quite high (25‐30%) in the bursa, chromosome analysis was used to assay for genome damage. Sister chromatid exchange frequencies of 3.9, 7.3, and 9.0 (baseline 2.5) per cell were obtained at MMS dosages per animal of 50 m̈g, 100 m̈g, and 200 m̈g, respectively. These results indicate the rapid and quantitative localization of DNA‐binding chemicals to cells of the bursa, particularly the resident B‐lymphocytes. The bursa should be a useful system for studying mutagen‐DNA interactions in the differentiating B‐lymphocyte and subsequent influences on the development of immunity and lymphoproliferative disease.