MODULATION OF THE SEROLOGICAL RESPONSE OF SPECIFIC PATHOGEN-FREE (EHV-FREE) FOALS TO EHV-1 BY PREVIOUS INFECTION WITH EHV-4 OR A TK-DELETION MUTANT OF EHV-1

EHV-1 was inoculated into specific pathogen-free (SPF) foals in order to study uncomplicated primary responses. Infection resulted in a stro ng serological response recognizing EHV-1-specific antigens; this cont rasts with a previous publication where a weak response was recorded i n SPF animals. Antibodies to EHV-1 were readily detected by four techn iques (virus neutralization, complement fixation, Western blots and im mune precipitation), yet there was comparatively little cross-reaction to EHV-4 target antigen. Re-inoculation with the same virus strain st imulated antibodies to EHV-1 but no additional antigens were recognize d and antibodies cross-reacting with EHV-4 antigens were not enhanced. Having characterized the uncomplicated primary response to EHV-1 in S PF foals, further animals were exposed to either EHV-4 or a thymidine kinase-deficient mutant of EHV-I prior to challenge with w/t EHV-1 to investigate how these infections might modulate the immune responses t o EHV-1 or 4. Primary inoculation with EHV-4 or with a thymidine kinas e-deficient mutant of EHV-1 produced productive infections as evidence d by virus shedding and pyrexia. In both these cases, however, in cont rast to that with w/t EHV-1, the serological response was very weak. R e-infection of foals primed with either EHV-4 or TK-deficient EHV-I wi th w/t EHV-I resulted in a strong response to EHV-I antigens detected by all four methods. In addition, in the foals given a primary inocula tion with EHV-4, superinfection with EHV-1 resulted in a strong cross- reactive response to EHV-4 target antigens. The relevance of these obs ervations to the interpretation of previously reported serological res ponses to EHVs in SPF and naturally reared animals is discussed.