Serial syngeneic transplantation of large granular lymphocyte leukemia in F344 rats

The legacy of the F344 rat as a cancer bioassay model (a retrospective summary of three common F344 rat neoplasms)

The Fischer 344 (F344) rat was used by the National Toxicology Program (NTP) for over 5 decades for toxicity and carcinogenicity studies. However, in 2006, the NTP decided to switch to a different rat stock due largely to high background control incidences of Leydig cell tumors (LCTs) and mononuclear cell leukemia (MNCL), also known as large granular lymphocytic (LGL) leukemia. In the current review, we aim (1) to provide a summary of NTP bioassays with treatment-associated effects involving MNCL and LCTs in addition to male F344-specific tunica vaginalis mesothelioma (TVM); (2) to describe important pathobiological differences between these F344 rat tumor responses and similar target tissue-tumor response in humans; and (3) to present the NTP reasons for switching away from the F344 rat. We show that due to the highly variable background incidence of F344 MNCL, more reliance on historical control data than is usual for most tumor responses is warranted to evaluate potential effect of any chemical treatment in this rat strain. The high spontaneous incidence of LCTs in the testes of male F344 rats has made this tumor endpoint of little practical use in identifying potential testicular carcinogenic responses. TVM responses in F344 rats have a biological plausible relationship to LCTs unlike TVM in humans. Given their high spontaneous background incidence and species-specific biology, we contend that MNCL and LCT, along with TVM responses, in F344 rat carcinogenicity studies are inappropriate tumor types for human health risk assessment and lack relevance in predicting human carcinogenicity.

Hepatic Lesions Caused by Large Granular Lymphocyte Leukemia in Fischer 344 Rats: Similar Morphologic Features and Morphogenesis to Those of Nodular Regenerative Hyperplasia (NRH) in the Human Liver

To characterize the hepatic lesions in Fischer 344 (F344) rats afflicted with large granular lymphocyte (LGL) leukemia, the livers of rats with LGL leukemia at various stages were examined histopathologically and immunohistochemically. The morphologic features in the livers of rats afflicted with LGL leukemia were diffuse, uniform-sized, granular, or micronodular lesions consisting of hepatocytes showing centrilobular atrophy and perilobular hypertrophy (CAPH) without fibrosis. With progression in the stage of the LGL leukemia, the severity of the CAPH of hepatocytes increased resulting in fatty change and/or single-cell necrosis, along with compensatory hyperplasia of the hepatocytes, finally resulting in lesions similar to those seen in nodular regenerative hyperplasia (NRH) in the human liver. The CAPH of hepatocytes was a nonspecific tissue adaptation against ischemia or hypoxemia and/or imbalance in blood supply due to disturbance in the portal circulation and hemolytic anemia induced by the leukemia cells. In addition, direct and/or indirect hepatocellular injuries by leukemia cells were considered to be necessary for the formation of human NRH-like lesions. Morphogenetic investigation of the livers of rats afflicted with LGL leukemia may be helpful to clarify the pathogenesis of NRH in the human liver.

A review of large granular lymphocytic leukemia in Fischer 344 rats as an initial step toward evaluating the implication of the endpoint to human cancer risk assessment

Large granular lymphocyte leukemia (LGLL) is a common fatal disease in aging F344 rats. The current understanding of rat LGLL and a search for mechanistic data/correlations to human leukemia were examined with the goal of improving evaluation of the LGLL endpoint in cancer bioassays as it relates to human cancer risk assessments. The exact cell of origin of the F344 rat LGLL is not fully resolved, although natural killer (NK) cell characteristics were demonstrated in most, if not all cases. Similarities between rat LGLL and a rare human NK-LGLL exist, invalidating claims of no human counterpart, although the underlying etiopathogenesis may be different. There is insufficient data to establish a mode of action of chemical-induced rat LGLL. Evaluation of the National Toxicology Program database revealed only 34 substances (out of over 500 studied) that were possibly associated with increased incidences of LGLL. Of these, only five produced definitive LGLL effects in both sexes; the remaining 29 produced single sex responses and/or only "equivocal" associations with LGLL. Trends of increasing background/variability in LGLL incidence and its modulation by extraneous factors (e.g., corn oil gavage) are key confounders in interpretation. Given that LGLL is a common tumor in control F344 rats, interpretations of bioassays can be improved by increasing the statistical stringency (e.g., p<0.01 over traditional p<0.05), as an indicator of possible carcinogenic effects, but that alone would be insufficient evidence for declaring treatment-related increases. Thus, it was concluded that the evaluation of possible chemically related increases in rat LGLL utilize a "weight-of-evidence" approach.

Histopathology of the spleen

The spleen contains hematopoietic and lymphoid elements, is a primary site of extramedullary hematopoiesis, and removes degenerate and aged red blood cells as well as particulate materials and circulating bacteria from the blood supply. Lesions of this important component of the immune system may center on the red pulp, the white pulp or involve both compartments The spleen is the site of direct and indirect toxicity, a target for some carcinogens, and also a site for metastatic neoplasia. Many systemic or generalized diseases have splenic involvement. This paper documents spontaneous background and treatment-induced lesions seen in rodent toxicity and carcinogenicity studies.

Chronic toxicity and oncogenicity study with glutaraldehyde dosed in the drinking water of Fischer 344 rats

Glutaraldehyde (GA) has a wide spectrum of industrial, scientific and biomedical applications. Its potential to produce chronic toxic and/or oncogenic effects was investigated in Fischer 344 rats (100/sex/group) given GA in drinking water for a maximum of 104 weeks. GA concentrations were 0 (control), 50,250 and 1000 ppm, resulting in average daily GA consumptions, respectively, of 0, 4, 17 and 64 mg/kg for males and 0, 6, 25 and 86 mg/kg for females. Interim euthanasia (10/sex/group) was performed at 52 and 78 weeks. Parameters evaluated were clinical signs, body weight, food and water consumption, hematology, serum chemistry, urinalysis, organ weights, gross and microscopic pathology. There were no treatment-related effects on mortality. Absolute body weights and body weight gains of the 250 and 1000 ppm males and females were reduced over the study in a dosage-related manner. Food and water consumption by the 250 and 1000 ppm groups were decreased in a statistically significant dose-related manner over the study, and mean water consumption by the 50 ppm animals was slightly reduced but not with statistical significance. The 250 and 1000 ppm groups had a dose-related decrease in urine volume with increased osmolality, and pH was slightly reduced. Absolute kidney weights were increased in the 250 and 1000 ppm groups at the 52 and 78 week sacrifices, and decreased at 104 weeks. Relative kidney weights were increased at all sacrifice times for the 1000 ppm group, at 52 weeks for the 250 ppm group, and at 72 weeks for the 50 ppm group. The urinalysis and renal weight changes are compatible with a physiological compensatory adaptation to reduced water consumption. Gross and histological evidence for gastric irritation was observed principally in the 1000 ppm rats euthanized at 104 weeks and in animals that died during the study. Bone marrow hyperplasia and renal tubular pigmentation, seen in rats that died and the 104 week euthanasia animals, may have been secondary to a low grade hemolytic anemia in animals with large granular lymphocytic leukemia (LGLL). The only neoplasm that showed a statistically significant increase was LGLL, which occurred at a high incidence in both sexes and all groups, including the controls, for both animals that died and at the 104 week euthanasia. A few instances of LGLL were observed at 78 weeks. The overall incidence of LGLL in the spleen for the 0, 50, 250 and 1000 ppm groups was, respectively, 43, 51, 40 and 46% for males, and 24, 41, 41 and 53% for females. Statistical analyses indicated that the severity of LGLL was associated with the higher dosages of GA in female, but not male, rats. Due to the background and variable incidence of LGLL in the Fischer 344 rat, the finding of a statistical significance only for female rats, and because, there was no clear dose-response relationship, the biological significance of the LGLL findings is unclear. There is the possibility that the significance was a statistical artifact due to the low incidence of LGLL in the female control animals as a result of biological variability within the study. It is also considered to be possible that the chronic dosage of GA in the drinking water resulted in a modification of one or more of the factors responsible for the expression of this common and spontaneously occurring neoplasm in the Fischer 344 rat.

Transplantation of large granular lymphocyte leukemia in congenitally athymic rats

Twenty-two congenitally athymic nude (rnu/rnu) rats were transplanted with large granular lymphocyte leukemia derived from F344 rats and then compared with ten similar rats inoculated with a suspension of normal F344 rat spleen cells. The normal spleen cells and tumor cells from a spontaneous, naturally occurring leukemia did not grow or cause clinical disease in any of the rats. All rats inoculated with a serially passaged leukemia cell inoculum had local growth at the inoculation site that spread widely and resulted in progressive tumor growth. Death occurred between 16 and 38 days after inoculation. The 22 rats that received passaged tumor cells developed leukemia and splenomegaly. Spleens were diffusely infiltrated by tumor cells and had severe depletion of lymphocytes in the white pulp. Leukemic rats were thrombocytopenic and had hemolytic anemia characterized by increased osmotic fragility, red cell width, and many nucleated erythrocytes. The disease syndrome appears similar to that of F344 rats transplanted with the same inoculum. Because the host rats lacked T cells, it is concluded that the hemolytic anemia and thrombocytopenia that develop in transplanted rats are independent of T cell function.

Effects of large granular lymphocyte leukemia on bone in F344 rats

Large granular lymphocyte (LGL) leukemia was induced in 40 F344 rats by inoculating them with neoplastic cells to evaluate the effect of acute leukemia on bone remodeling and calcium balance. The rats developed leukemia and splenomegaly by 9 days after inoculation. The rats had reduced body weight (day 12), food intake (days 4, 8, 12), urine production (day 12), and fecal output (day 12). Serum calcium and phosphorus and urinary excretion of calcium and phosphorus were decreased on days 8 and 12 in leukemic rats. Static bone histomorphometry of trabecular bone in lumbar vertebrae demonstrated reduced bone area, no change in the number of osteoclasts, and reduced osteoclast perimeter at day 12. Dynamic bone histomorphometry revealed reduced double labeled perimeter, mineralizing perimeter, trabecular mineral appositional rate, and bone formation rate in rats with LGL leukemia at days 9 and 12. There was no change in periosteal mineral appositional rate. Rats with leukemia and intramedullary neoplastic cells had a reduction in bone formation rate that resulted in a loss of trabecular bone.