Mycoplasma pulmonis and Lymphoma in Bioassays in Rats

Abstract

Abstract. Lymphomas were reported to be induced in rats in bioassays of aspartame, methyltertiary-butyl ether (MTBE), and other chemicals conducted by a nonprofit cancer research organization. European regulatory authorities concluded that lymphomas in the aspartame study were caused by Mycoplasma pulmonis and suggested that this also was the case for the MTBE bioassay. To assess the role of M. pulmonis in these bioassays, we reviewed the tumor data for the aspartame and MTBE bioassays and, additionally, the organization's bioassay of methanol. For all 3 studies, the most frequently reported hematopoietic neoplasm was lympho-immunoblastic lymphoma, the most frequently affected organ was the lung, and, in almost half of the rats with this diagnosis, the lung was the only affected organ. Lesions diagnosed as lymphoma in published illustrations had pleomorphic cellular morphology and appeared to contain neutrophils. Information from these reports and other sources indicated that lesions typical of M. pulmonis disease were prevalent among the aspartame and MTBE study rats and that the rats were not specific-pathogen-free. Because the lymphoma type, cellular morphology, and organ distribution reported in these studies are atypical of lymphoma in rats, because lymphocyte and plasma cell accumulation in the lung is characteristic of M. pulmonis disease, and because M. pulmonis disease can be exacerbated by experimental manipulations, including chemical treatment, we suggest that a plausible alternative explanation for the reported results of these bioassays is that the studies were confounded by M. pulmonis disease and that lesions of the disease were interpreted as lymphoma.

Commentary

“…13 We concluded that the rats used in these bioassays probably had Mycoplasma pulmonis disease, that the primary form of lymphoma reported is inconsistent with recognized forms of lymphoma in rats, and that lesions of M pulmonis disease were plausibly interpreted as lymphoma. We subsequently obtained and analyzed the nonneoplastic lesion data for these studies, analyzed the age distribution of lymphoimmunoblastic lymphoma, and accessed an excerpt of a court deposition regarding serologic testing for mycoplasma infection in the organization's rat colony.…”

“…13 We therefore evaluated occurrence of inflammatory lesions in organs other than the respiratory tract and ear and found that such lesions were unexpectedly prevalent among the rats used in these 3 studies, as shown in Table 2. Of all 2,960 rats in the 3 bioassays, 2,462 (83.2%) were listed as having chronic inflammation of some combination of mandibular, axillary, inguinal, and/or mesenteric lymph nodes.…”

The carcinogenic effects of aspartame: The urgent need for regulatory re‐evaluation

“…They considered the RI studies to have several methodological "flaws," such as: (1) the inappropriately large number of animals per sex and per group, the numerous doses tested, prenatal exposure, and the life-span treatment and observation of the animals, the claim being that the study design and conduct are in contrast with current guidelines of the Organization for Economic Cooperation and Development (OECD) [2009] and other international protocols that recommend the use of adult animals (6-8 weeks old) at the start of studies, duration of studies limited to 24 months to avoid high background tumors in controls (which may affect the ability to evaluate the significance of small increased incidences of tumors in the tested groups) [EFSA, 2006[EFSA, , 2009; (2) high background infection in the RI rat colony allegedly affecting survival and tumor rates [EFSA, 2006;FDA, 2007;Hayes et al, 2011]; (3) uncertainty about the "correctness" of diagnosis of some tumor types, in particular lymphomas/leukemias [EFSA, 2006;Schoeb et al, 2009]; and (4) a lack of relevance to human risk assessment in the cases of statistically significant dose-related increased incidences of hepatocellular carcinomas (P < 0.05) and lung carcinomas (P < 0.05) observed in male Swiss mice that were induced by non-genotoxic agents EFSA, 2011a; EFSA, 2011b].…”

Mycoplasma Pulmonis and Lymphoma in a Methanol Bioassay

“…9 We consider it in the public interest to promote resolution of the scientific questions that have been raised about the methanol and other bioassays. 2,4,[6][7][8][10][11][12][13]15,16 We therefore obtained the results of the slide review from the National Institute of Environmental Health Sciences via a Freedom of Information Act request and analyzed them for differences between the NTP pathologists' diagnoses and those of the study pathologist. We tabulated diagnoses of lymphoma, leukemia, and histiocytic sarcoma in thymus, lung, liver, spleen, and lymph nodes and inflammatory lesions in the nose, lung, and ear.…”