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The Effect of Immunization on the Fate of Injected Second Stage Ascaris Lumbricoides Larvae in the Rabbit^*

Albino rabbits immunized with lyophilized, in vitro hatched, second-stage Ascaris larvae and nonimmune rabbits were challenged intravenously with a suspension of live "in vitro" hatched larvae. Those injected into immune animals were immobilized in septal capillaries of the lung and degenerated rapidly. As a result, few hemorrhages were noted in the lungs. On the other hand, larvae injected into control animals migrated readily to the interstitium and alveolar spaces causing extensive hemorrhages. Most of the larvae were found in the alveolar sacs of control animals as late as 12 days after inoculation; of these, many had developed into advanced-stage larvae. No larvae were found in immune animals after 4 days.

Although the inflammatory response appeared quickly in both immune and nonimmune rabbits, it was more intense in the former. The acute inflammatory reaction was characterized by accumulations of neutrophils and eosinophils. This was later replaced by a granulomatous process containing abundant epithelioid cells, multinucleated giant cells and, in the immune, by the presence about degenerating larvae of an intensely acidophilic material intimately related to the cuticle of the larvae. This material, which is probably an antigen-antibody complex, was never observed in control animals. The granulomatous process lagged by 24 to 48 hours in the controls as compared with the immune animals. Resorption of the granulomas was almost complete 20 days after challenge in the immunes and had markedly decreased in the controls 30 days postinoculation. It is suggested that the X-ray findings described in the so-called "eosinophilic pneumonia of ascariasis" may be the result of a granulomatous process rather than being due entirely to accumulations of eosinophilic granulocytes.

The rapidity with which the acute and the granulomatous inflammatory responses appear both in the immune and control animals indicate that mechanisms other than hypersensitivity may play a role in the defense against Ascaris larvae. There was no relationship between the levels of circulating antibody and the severity of the lesions found. No decrease in circulating antibody was observed after the challenging dose was administered.

^* This study was supported by Research Grant E-3212 of the National Institute of Health, Education and Welfare, Bethesda, Maryland.