Comparative Studies on Host Responses to Shope Fibroma Virus in Adult and Newborn Rabbits2

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Abstract
Intradermal inoculation of Shope fibroma virus into adult and 4-day-old New Zealand white rabbits resulted in the formation of typical fibromas. In adult rabbits, tumors regressed in a few weeks, but in newborns, they grew larger and either killed the animals or regressed slowly. Histologically, adult rabbit tumors contained numerous mononuclear inflammatory cells and vascular changes characteristic of an Arthus reaction. In contrast, newborn animal tumors had few inflammatory cells and no vascular changes. The amount of fibroma virus associated with the tumors of newborn and adult rabbits was quantitatively different. Over 100-fold more infectious virus was found in tumors of newborn rabbits. In both adults and newborns, maximum titers of virus occurred on the 6th day after virus inoculation. Infectious virus could be recovered longer from tumors of newborns. Tumors of adult rabbits had higher levels of interferon-like activity. Ma,ximum interferon levels were demonstrated 2 days after virus inoculation but could no longer be detected before maximum tumor growth. Neutralizing antibody responses to fibroma virus and antibody responses to surface antigens of fibroma virus-infected cells, measured by indirect immunofluorescence and 51Cr-release, were similar in adults and newborns. Cellular immunity to fibroma virus was demonstrated in newborn and adult rabbits by inhibition of migration of peritoneal exudate cells. Newborns, under conditions of tumor growth, do not seem to have a significantly impaired humoral immune response to fibroma virus, nor do the lymphocytes from these animals lack the ability to produce migration inhibitory factor when exposed to viral antigens.

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