Introduction
In 1981 the Acquired Immune Deficiency Syndrome (AIDS) was firstly identified as a new disease caused by a retrovirus, human immunodeficiency virus type 1 (HIV-1). It was the cause factor of the most devastating disease that emerged in the last 35 years. HIV-1 also spreads by percutaneous and perinatal routes, or exposure at mucosal surfaces, but primarily via sexual transmission.(Cohen et al., 2011) Since the identification of this virus it infected more than 60 million people and caused about 25 million deaths. Developing countries have the highest mortality rate with especially high numbers in sub-Saharan Africa.
The sudden emergence of this new virus has always been in the focus of study of researchers. In 1986 an antigenically different HIV type 2 has been discovered in western Africa.(Clavel et al., 1986) This is very closely related to a simian virus commonly found in macaques.(Chakrabarti et al., 1987) After that, more simian immunodeficiency viruses (SIVs) were found in different species of primates for example in chimpanzees(Huet et al., 1990) and sooty mangabeys(Hirsch et al., 1989) , which were close relatives to HIV-1 and HIV-2. These viruses are mostly non-pathogenic to their hosts. These evidences implied the idea that AIDS developed in both macaques and humans because of cross-species infections with lentiviruses from different primate species.(Sharp et al., 1994) Moreover, HIV-1 and 2 are most likely a result of virus transfer to humans from infected primates in Africa.(Hahn et al., 2000) In this essay, I am going to summarize the known facts about the precursors of HIV-1 and HIV-2, examine their most likely evolutionary history and go through the steps that leaded to the AIDS pandemic.
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Primate lentiviruses
Primate lentiviruses are mostly transmitted horizontally, but recent studies came up with evidence of vertical transmission (when they also infect the germ cells of the host) that occurred many times e.g. in Malagasy lemurs about 4.2 million years ago.(Gilbert et al., 2009) These living species provide direct evolutionary evidence of lentivirus changes. Molecular data analysis suggested that the ancestral SIVs existed a few hundred years ago. Only apes and monkeys from Africa were infected with SIV. This might suggest that the primate lentivirus emerged in Africa before the division of the Asian and African Old Word Monkeys, about 6-10 million years ago. However, neither Asian nor African New Word Monkeys were tested for the endogenous SIVs, therefore we cannot have a clear understanding of primate lentiviruses’ evolutionary history. It became clear though that SIVs are spread among 40 species of primates, most infections happened within species but also numerous cross-species transfer took place, generating newly emerged mosaic virus lineages. (Jin et al., 1994)
SIVcpz, a lineage within simian viruses, has quite similar genetic properties as HIV-1 and they show a close genetic relationship. Interestingly, only two of the common chimpanzees have this lineage of the virus, which indicates that these 2 species had acquired the SIVcpz more recently, after the split of their lineages.(Sharp & Hahn, 2011) According to the phylogenetic analysis, SIVcpz shows a complex mosaic. This is a result of a recombination of two ancestral SIV lineages, which might have occurred during predation (chimpanzees hunt for mammals and other monkeys) and lead to cross-species transmission event. (Bailes et al., 2003)
SIVcpz compared to other SIVs are pathogenic to its host and spread through sexual routes. They also increase the risk of mortality in infected individuals. Similarly to HIV-1, they cause CD4+ T-cell loss and can lead to the end stages of AIDS. Overall, they have a negative impact on reproduction and health of infected individuals.
A recently found new lineage (SIVgor) within the radiation of SIVcpz, appeared in several gorillas. It happened possibly due to a single cross-species infection from chimpanzee. Phylogenetic analysis supported this idea.(Takehisa et al., 2008) Due to the fact that only a few gorillas are infected it is hard to study SIVgor, but it has similar effects as HIV-1 on infected individuals (e.g. CD4+ T-cell loss).
HIV-1 origin
HIV-1 is further subdivided to M, N, O, and P groups that represent independent cross-species transmission events. Group M were recognised firstly, and had the most devastating effect on human populations among the 4 group. It causes the death of millions of people and has an impact in all countries on the Earth. Group O had been identified in 1990, and responsible for only about 1% of HIV-1 infections. Patients came from the area of Cameroon and Gabon. 8 years later a new N type was found, and so far, appeared only in 13 cases. The recently discovered (2009) P type only effects 2 individuals, a Cameroonian woman who lives in France and another person from Cameroon.(Plantier et al., 2009) In all types of this virus there is CD4+ T-cell depletion and the AIDS disease appears.
N and M type are more closely related to SIVcpzPtt, which indicates that the origin of these 2 types might be from chimpanzees. Furthermore, it was possible to even identify their ape precursors. M type most likely emerged in the area of Sangha and Boumba rivers, while N type possibly originates from the Dja Forest, both found in Cameroon.(Heuverswyn et al., 2007) The gorilla origin of P type is supported by existing phylogenetic data, however, some characteristic of the SIVgor trait also can be observed in this group. Since there is no particularly closely related ape virus to the O group, the origin of it is still unknown. The fact that O and P is more closely related to SIVcpzPtt than to SIVcpzPts implies that these 2 groups originate from west-central Africa.
The way how humans acquired all the types of HIV-1 is still not known. However, some suggestions can be made based on the biology and spreading mechanism of these viruses. The transmission must have occurred through mucous or cutaneous membrane exposure to infected ape body fluids and/or blood. Whatever the way it happened, the ape-human interactions resulted in 4 independent cross-species transmission event in west-central Africa.
HIV-2 origin
HIV-2 has remained mostly in West Africa since it has been discovered in 1986. Interestingly, the HIV-2 infection does not result in AIDS disease mostly, but when it does, it causes identical symptoms as HIV-1. Firstly, a sooty mangabey origin of HIV-2 was suggested in 1989(Hirsch et al., 1989). HIV-2 was very similar to a locally spread SIVsmm infection among sooty mangabeys supporting the idea mentioned previously. Moreover, these kinds of mangabeys were hunted for agricultural pets in this area of West Africa providing a possible way of cross-species infection. 8 different subgroups of HIV-2 viruses (A-H) had emerged so far, which are an analogue to HIV-1 viruses. A and B have an influence on a large group of people while the other 6 only have an impact on few individuals. All the 8 groups originated from the area of Cote d’Ivore, Liberia and Sierra Leone. (Sharp & Hahn, 2011)
Adaptation to new host
To gain access and to became able to infect new species, all virus trait must get over barriers of the host immune system. To become successful in such challenges, they need to overcome some adaptive hurdles before a lentivirus can effectively infect other species like humans. Evidence of host-specific adaptation had been found in change of sites in viral proteome. The viral matrix protein (Gag-30) encoded a Met in both SIVgor and SIVcpzPtt traits, while this had been switched to express Arg in HIV-1 ancestor, and it had been conserved as a basic amino acid (Arg, Lys).(Wain et al., 2007) This type is more efficient in humans while the ancestral trait is more successful in apes.
Origin of AIDS
Phylogenetical and statistical analysis dated the last common ancestor of HIV-1 M to 1910-1930, so it has been spread for 50-70 years before it had been discovered. Molecular epidemiological studies even identified the town where it probably originated, Leopoldville in west-central Africa. There were lineages only found in this area and also all subtypes of M had been found there. Leopoldville was the largest city in the area at that time, a likely destination of a newly emerged virus to spread. (Worobey et al., 2008)
Conclusion
Recent studies helped in understanding the evolution and the most likely origins of SIV and HIV. The method of transmission from primates to human is still unknown, but possible explanations can be made according to the biology of these viruses. Different types of HIV viruses are most likely a result of numerous cross-species transmissions from gorillas and chimpanzees to humans that lead to one of the most devastating diseases in the last 35 years.
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