Use of Semliki forest virus to identify lipid-mediated antiviral activity and anti-alphavirus immunoglobulin A in human milk.

In a simple and reliable assay system, Semliki forest virus (SFV) was used to detect the activity of antiviral factors in human milk. Fractionation of the milk showed that a heat-stable, lipid-associated activity and an immunoglobulin-associated activity were present, either singly or together, in 85% of the human milk samples tested. Cow and synthetic milk showed neither activity. Extraction of the neutral milk lipids allowed the antiviral to be located with the monoglyceride and free fatty acid fractions. The milk low in antiviral lipids and high in triglycerides also lacked a strong lipase activity. The immunoglobulin anti-SFV activity was shown to be due to immunoglobulin A, the major milk immunoglobulin, and appears to be directed against an alphavirus closely related to SFV, possibly Ross River virus.