The antagonistic effects of positive and negative lipids were evident in the study of their action upon viruses. Generally, agents with a positive polar group appeared to create favorable conditions for the development of viruses, while those with a negative polar group had an opposite effect. This influence, which was first seen in phages in vitro, became still more evident in viral infections.

Subcutaneous administration of positive lipids, such as sterols or insaponifiable fractions of organs, induced greater local receptivity to viruses. In experiments with smallpox virus in rabbits, for instance, virus inoculation of the skin induced an exaggerated response in those areas where positive lipids previously had been injected subcutaneously compared with the response in other previously untreated areas. In less sensitive species such as mice and rats, positive lipid injections induced abnormally high local receptivity to virus inoculation. Intracerebral injection of sterols followed by subcutaneous inoculation with smallpox virus invariably produced nervous system localization of the virus. Intraperitoneal administration of sterols in very high doses in mice prior to smallpox inoculation produced a great degree of central nervous system localization. Intracerebral virus inoculation, after subcutaneous administration of high doses of anti fatty acids, brought death earlier in test animals than in controls given intracerebral virus alone.

A striking opposite effect was noted for lipids with a negative polar character. In rabbits, subcutaneous injection of a polyunsaturated fatty acid set up a local skin area refractory to smallpox virus inoculation, although inoculation was positive in other areas of the body. Death also occurred later, following intracerebral inoculation with a neurotropic virus in test animals given subcutaneous or intraperitoneal injections of fatty acids, than in controls. This partially protective effect was opposite to the increased receptivity seen in animals injected subcutaneously or intra peritoneally with insaponifiable fractions and intracerebrally with the same virus, where death appeared earlier than in controls.

The antagonistic effects of the two groups of lipids for viral infection appeared interesting from several points of view. The effects were local, at the cellular level, where viruses themselves act. Subcutaneous injection of lipids induced manifest changes in response toward the virus in the skin at the site of injection, and little or no change at all elsewhere. We have utilized this fact, as we will see below, to obtain information regarding the level at which various agents act. A change induced in receptivity to viruses, limited to the skin at the site of injection, would indicate activity of the agent at the cellular level. Tests based upon the skin response to smallpox virus infection have shown that, among the lipids with a negative polar character, a maximum of influence is exerted by the insaponifiable fraction of organs of exodermic origin from species sensitive to the virus. The insaponifiable fractions of rabbit skin and rabbit brain were the most active of the lipids tested. Among the fatty acids, the preventive effect was seen to increase with the degree of desaturation. It was almost entirely absent in saturated fatty acids, notably present in polyunsaturated fatty acids.

The increase and decrease in receptivity of the skin to smallpox virus following injection of lipids also furnished information about the roles of the polar and nonpolar parts of lipids in this specific activity. An opposite effect was seen between two groups of substances having the same non polar group but differing in their polar groups. While the polyunsaturated fatty acids of safflower oil, for instance, greatly reduced receptivity, the same polyunsaturated members having alcohols as polar groups increased receptivity. The polar group—negative or positive—appears to be the factor inducing the opposite effect.

The role of the nonpolar group was studied by comparing saturated and unsaturated acids and alcohols. Almost no activity was seen for the saturated. The unsaturated members were active in general, with activity in any direction increasing with the degree of desaturation of the nonpolar group. Thus, it appears that the nonpolar group determines whether a substance is active or inactive, but the nature of the activity—that is, increasing or decreasing receptivity—is determined by the polar group.

The influence exerted by agents with a positive character upon viral infection would explain the seasonal changes in clinical manifestations which are especially interesting for the paralytic form of poliomyelitis.

We could show experimentally that when mice, after being inoculated subcutaneously with smallpox vaccine virus, are kept in an incubator at 37°C, all develop cerebral involvement, while such involvement appears in only a small proportion of other animals kept at room temperature, and does not appear at all in those kept in a cool place. As we could also show that one of the effects of exposure of an animal to a higher temperature is an increase in the body of the amount of free lipids with positive character, this could explain the increase in the virus sensitivity of cells in the central nervous system which are especially sensitive to these lipids. This relationship would also explain the increased incidence of paralytic polio cases during hot weather.

The presence of greater amount of lipids with positive character in youth helps also to explain the frequency and intensity of viral infections in children. (Note 31)

The study of the effects of temperature and lipids upon viruses has shown that those effects are not limited to the host but also are exerted upon the viruses. (Note 32) The influence of heat and cold upon virus activity was studied in bacteriophages, where effects for virus and host could be separated. The direct influence upon the virus appeared relatively small and secondary to the changes which appear in the host itself. Bacteriophage, separated from microbes by nitration and kept in an incubator at temperatures 2-3 degrees C higher or lower than controls, showed no change in virulence. This was true as long as microbes were not present. Microbes kept at higher temperature were more sensitive to phages; when kept at lower temperature, they were less sensitive. This influence went so far as to change a sensitive strain to a refractory one, and vice versa.

The fact that microbes grown at higher temperatures favor the development of bacteriophage while those grown at lower temperature hinder it could be correlated with the change in the richness of lipids in the microbes themselves. Similar results were obtained when microbes were grown for a time in media containing fatty acids or insaponifiable fractions and were then removed and exposed to phages. These experiments (Note 33) indicate the direct role played by the lipids of the hosts in the activity of phages, and would explain the influence exerted by temperature. Through the change in the lipids of the host, the virus changes too, becoming more active if grown in microbes at a higher temperature and less aggressive if passed through microbes kept at lower temperature.