INTERACTIONS BETWEEN EXOGENOUS AND ENDOGENOUS RETROVIRUSES

Retroviruses are distinguished from other viruses by several features. Notably, some retroviruses are present as normal elements in the geno mes of virtually all vertebrates (endogenous proviruses). Others are e xogenous, i.e. horizontally transmitted agents, many of which cause fa tal diseases. The endogenous retroviruses are genetically transmitted and to a large extent their significance is uncertain. However, there is evidence suggesting that they contribute to the development of dise ases in several animal species. Most importantly, some endogenous retr oviruses are capable of interacting with exogenous counterparts throug h a variety of different mechanisms with serious consequences to the h ost. Conversely, others are advantageous in that they protect against exogenous retroviruses. In this review various types of interactions b etween endogenous and exogenous retroviruses are discussed, including receptor interference, recombination, phenotypic mixing, immunological interactions and heterologous trans-activation.