Monoclonal antibodies (Mabs) are prepared by immunizing an animal (generally a mouse or rat) and fusing the spleen cells, after a given period of time, with a plasmacytoma cell line (Köhler and Milstein, 1975). The fusion mixture is plated out into such numbers of microcultures that the likelihood of obtaining cultures with single fusion events is reasonable (De Blas et al., 1981). Supernatants from individual cultures are tested with a suitable immunoassay for the presence of antibodies with the desired specificity.

The target antigen of the FN18 monoclonal antibody, called RhT3, is probably the rhesus monkey homologue of the human CD3 antigen, expressed on mature T cells. RhT3 appears to be polymorphic, since FN18 was not reactive with T cells from all the screened animals. Thus, immunofluorescent staining of peripheral blood lymphocytes with FN18 antibody revealed either a positive or a negative phenotype for the target antigen. In a rhesus monkey population, nonreactivity for FN18 was observed in low frequency (2.7%). Expression and non-expression of RhT3 appeared to be constant characteristics. Non-expression was not associated with any demonstrable immunodeficiency. Further, there seemed to be no association between the presence of the FN18 target antigen and sex, age or expression of MHC class I antigens. Family studies indicated that the positive phenotype is expressed in the same fashion by animals presumably heterozygous or homozygous for the positive allele, the negative phenotype being expressed only on cells from animals homozygous for an assumed blank allele. Therefore, the positive phenotype is likely to be transmitted in an autosomal dominant mode. In the animals with the negative phenotype, normal T-cell numbers were present, as was demonstrated by B- and T-cell specific monoclonal antibodies. Cytoplasmic staining of cytospun lymphocyte preparations with FN18 revealed that polymorphism was present at the intracellular level. Cell proliferation tests, using PWM and Con A as mitogens, showed the presence of apparently functional RhT3 cell surface molecules in FN18 non-reactive animals. Polymorphism is therefore assumed to be at the epitope level.

Fluorescein (Fl) and tetramethyl rhodamine (Rh) were evaluated as possible candidates for a double hapten sandwich system in enzyme immunohistology. Monoclonal antibodies were raised against Fl and Rh. Their fine-specificity was tested with a competition-like assay. A pair of Mab's was selected for immunohistology in which they functioned as a bridge between Fl/Rh conjugated antibodies and Fl/Rh labeled peroxidase and alkaline phosphatase, respectively. The binding of fluorescein labeled antibodies could be successfully demonstrated in histological slides. A large variability in the efficacy of staining was observed in the case of rhodamine labeled antibodies. The phenomenon is explained by assuming that tetramethyl rhodamine isothiocyanate reacts preferentially with lysine residues near to, or embedded in, hydrophobic regions in a protein. This condition may reduce the accessibility of the Rh moiety for anti-Rh antibodies.

An attempt is made to relate the presence of autoantibodies to pathological changes in aged Mastomys. It appeared that a direct relationship between one of the thymic changes (atrophy, hyperplasia, or neoplasia) and autoantibody formation was doubtful. An association was found between autoantibodies and tumors involving the lymphoreticular tissues (excluding the thymus). Animals in which the lymphoreticular tissues were entirely replaced by tumor cells did not have autoantibodies. For the other lesions, which included thyroiditis, myositis, myocarditis, immune complex glomerulonephritis, and vascular lesions, a direct relationship was found only between thyroiditis and autoantibodies to colloid. The conclusion is that Mastomys seems to be more suited as for studying general phenomena underlying autoimmunity than for studying specific autoimmune diseases.

The occurrence of lymphoplasmacellular thyroiditis associated with the presence of autoantibodies to thyroid colloid in Mastomys makes it likely that there is an autoimmune thyroiditis in this animal species. Thyroiditis occurred in 16% of the males and in 23% of the females. From longitudinal studies performed in a number of the animals, it appeared that the autoantibodies occurred from the age of 13 months onward and that they persisted until the death of the animals in the majority of the cases. The serum concentration of antibodies to thyroid colloid in Mastomys correlated only roughly with the extent of thyroid infiltration. Neither differences in serum thyroxin concentration nor in proliferative thymic lesions were found between animals with and without thyroid disease.

One hundred and thirty-five sera from aged Mastomys were screened for the presence of spontaneously occurring autoantibodies using an indirect immunofluorescence technique. Such antibodies appeared to be relatively frequent and a number of different tissue specificities were observed. Autoantibodies observed were antibodies to cytoplasmic antigens in liver cells, thyroid follicular cells, and renal proximal tubular cells, striated and smooth muscle, gastric parietal cells, nuclear factors, colloid, and erythrocytes. Several antibody specificities were present in most sera, with a maximum of six and a median value of three different specificities per Mastomys. Anti-striational antibodies of the myasthenic type and anti-colloid antibodies belonged to the IgG class, antierythrocyte antibodies to the IgG and IgM classes, and the other types to the IgM class. From preliminary histopathological data it appears that of the spectrum of autoantibodies present in Mastomys, only autoantibodies directed to colloid could be associated with distinct histopathological changes (i.e., lymphoplasmacellular thyroiditis). Although striational antibodies as found in patients with myasthenia gravis were common in Mastomys, clinical signs of increased muscular weakness were lacking and antibodies to acetylcholine receptors were absent. The multiplicity of autoantibodies in this species together with other abnormalities in the immune system make Mastomys a promising model for the study of basic mechanisms underlying autoimmunity.

In our studies on a possible role of type C oncoviruses in human leukemia we applied the technique of cocultivation of human bone marrow with an animal indicator cell line. Dog thymus A7573 cells were cocultivated with human bone marrow samples from normal individuals, leukemic patients, and nonleukemic patients. By means of the indirect cytoplasmic immunofluorescence assay (IFA), antigens which crossreacted with the major internal protein (p30) of the woolly monkey (simian) sarcoma leukemia virus (SiSV) complex could be detected (Nooter et al. 1979). In the case of childhood leukemia, five out of nine cocultures showed virus-related IFA staining.

Praomys (Mastomys) natalensis develops a wide variety of autoantibody specificities with age; of these only those to thyroid colloid were associated with a histopathological lesion e.g. thyroiditis. An inverse relationship was found between the mean number of autoantibodies and the occurrence of lymphoreticular tumors. It appears that Mastomys is a better model for studies on the general mechanisms of autoimmunity than for investigating specific autoimmune diseases per se.

Bone marrow of leukaemic patients, non-leukaemic patients and normal individuals were co-cultivated with the canine cell line A7573. These co-cultures were screened for retrovirus antigens by means of the indirect cytoplasmic immunofluorescence assay (IFA). Rabbit antisera directed against the major structural protein (p30) of woolly monkey (simian) sarcoma leukaemia virus (grown in human lymphoid cells) and Rauscher murine leukaemia virus were used for testing. After 2 months in culture, 6 of 17 co-cultures containing cells from leukaemic patients showed positive staining in the IFA with the anti-simian virus serum. In control dog cells fluorescence was never observed. Five of the six positive cultures were derived from leukaemic children. One of 12 co-cultures of the non-leukaemic group and one of nine normal bone marrow co-cultures were positive with the simian virus antiserum. None of the 38 co-cultures stained positive in the IFA with Rauscher virus antiserum. Absorption of the simian virus antiserum with calf serum or mouse mammary tumour virus had no dramatic effect in the IFA on positive control cells or on cells of a positive co-culture. However, absorption with purified simian virus (grown in rat cells) completely abolished these fluorescence reactions. The results provide evidence that simian sarcoma-leukaemia virus related information was present in the original bone marrow samples and that co-cultivation with permissive mammalian cells enabled the detection of virus footprints.

Rabbit corneal cells transformed by a putative human type‐C helper virus pseudotype of the mouse sarcoma virus produce large amounts of transforming and non‐transforming viruses. The virions are antigenically related to the woolly monkey (simian) sarcoma‐leukemia type‐C oncovirus. Typical sarcoma virus lesions developed in newborn rats injected with virus‐producing rabbit cells. Cells producing only the putative type‐C helper viruses as a result of exposure to a high dilution of transforming virus stock induce lymphosarcomas after inoculation into newborn rats.