Herpes simplex virus type 1 (HSV‐1) expresses a complex of two virally encoded glycoproteins, gE and gI, which is capable of binding nonimmune human IgG. The gE‐gI complex has thus become known as an Fc receptor (FcR), which reportedly binds human IgG subclasses in the order IgG4 > IgG1 ≥ IgG2 and does not bind IgG3 from many individuals. There is, however, allelic variation in the genes encoding the human IgG1 heavy chain constant region and this gives rise to allotypes of IgG1. Using recombinant monoclonal IgG molecules of known isotype and mutants thereof we have unexpectedly discovered that the HSV‐1 FcR discriminates between IgG1 allotypes. This is evidence of functional differences between IgG1 allotypes that may account for their distribution in populations. Furthermore, these findings suggest HSV‐1 FcR binding sites on the IgG molecule some distance from the proposed binding site in the CH2‐CH3 domain interface.

A virally encoded, high-affinity Fc receptor (FcR) is found on herpes simplex virus type 1 (HSV-1) particles and infected cells where its binding of non-immune IgG protects cells from host-mediated lysis. Whilst mutation or aglycosylation of the IgG CH2 domain reduced binding to human FcR, the interaction with HSV-1 FcR was not affected. However, the HSV-1 FcR, unlike human FcR, discriminates between human IgG1 allotypes, being sensitive to changes at positions 214 (CH1) and 356/358 (CH3), away from its proposed binding site at the CH2-CH3 interface. The biological consequences are not known but this is the first evidence of a major functional difference between IgG1 allotypes.

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