Susceptibility to Lymphoma Induction by Gross' Passage A Virus in C3Hf/Bi Mice of Different Ages: Relation to Thymic Cell Multiplication and Differentiation2

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Abstract
Gross' Passage A virus is most likely to induce lymphomas in C3Hf/Bi mice if it is injected early in life; the virus is much less effective later. To find out if the thymus plays a role in determining these reactions to the virus at different ages, a curve was established relating the incidence of lymphoma and the survival time of C3Hf/Bi mice to age at the time of virus administration, and thymuses of normal C3Hf/Bi mice of different ages were examined for any age changes in this organ that could be correlated with the curve of susceptibility to the virus. Susceptibility was greatest during the first 2 weeks of life when the total number of cells in the thymus was increasing to 30 or more times its value at birth. In the outer cortex of the thymus at this time, a zone of large, relatively undifferentiated, multiplying cells was prominent and many of these cells were under going differentiation to smaller multiplying lymphocytes. Susceptibility decreased simultaneously with completion of thymus growth when the outer cortical zone became narrowed due to diminution in the number of these large, relatively undifferentiated cells. Susceptibility of C3Hf/Bi mice to induction of lymphoma by Passage A virus is thus associated with, and may depend on, the presence of large numbers of relatively undifferentiated cells that are proliferating and differentiating in the cortex of the thymus; decrease in susceptibility with age appears to be due to the decrease in the number of these cells as the animals grow older. It is suggested that the virus starts the lymphoma induction process by interfering, directly or indirectly, with differentiation of thymic cells without preventing their multiplication.

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