Classification of HIV

The acquired immunodeficiency syndrome (AIDS) is caused by two different human immunodeficiency viruses, HIV-1 and HIV-2, which belong to the lentivirus family of retroviruses. Another large group of related lentiviruses has been discovered in nonhuman primates, and designated as simian immunodeficiency viruses (SIV). All known primate lentiviruses are grouped into five distinct phylogenetic lineages. One of these lineages includes all HIV-1 isolates, as well as viruses from chimpanzees (SIVCPZ). A second lineage includes multiple strains of HIV-2, and viruses isolated from sooty mangabeys (SIVMN) and from captive macaques (SIVMAC). The other three lineages include viruses isolated from different species of African monkeys [7-9].

Phylogenetic analyses of the nucleotide sequences of the envelope (env) and core (gag) genes of a large number of HIV-1 isolates, have identified the existence of at least eight different genetic subtypes (clades) of HIV-1, which have been designated A through H [10-16]. These genetic subtypes, which form the major group of HIV-1 (group M), are approximately equidistant to each other, although subtypes B and D seem to be more closely related. A second group of HIV-1 isolates, distantly related to group M viruses, has been recently identified in patients from Cameroon, and designated as group O viruses (for "outliers"). This group of viruses is very heterogeneous, and in fact it may comprise a number of distinct genetic subtypes, which remain to be identified and characterized [17,18].

It is important to indicate, however, that these genetic subtypes, or clades, are not necessarily equivalent to antigenic or immunological subtypes, and that at the present time it is not known what could be their relevance to vaccine-induced protection.