Researchers studying wild Gombe chimpanzees infected with an HIV-predecessor virus have discovered that our close relatives may carry a gene that helps to lessen the impact of the viral infection. Furthermore, this gene bears a striking resemblance to a human gene that delays the progression of HIV by reducing the viral load. The findings were published in PLoS Biology this week. 

A team led by Stanford’s Emily Wroblewski and Peter Parham analyzed DNA from a well-studied population of wild chimpanzees living in Gombe National Park in northwestern Tanzania. These chimps are naturally infected with SIVcpz, a strain of the simian immunodeficiency virus. SIVs are precursors to HIV-1 and HIV-2. Because SIVs are so diverse, these viral infections can be nonpathogenic, which may represent an evolutionary adaption between primates and their viruses. 

In humans and chimps alike, cell-surface proteins called major histocompatibility complex (MHC) class I molecules recognize foreign invaders and activate the body’s immune responses to viral infections. With viruses evolving so rapidly, the MHC class I molecules have evolved rapidly in response, becoming extremely diverse and species-specific. Using DNA from fecal samples, the team sequenced an MHC gene called Patr-B from 125 Gombe chimps. They were able to identify 11 Patr-B alleles, or variants of the gene. 

Over a 15-year period, two social communities (out of three) flourished, maintaining one or two high-frequency Patr-B variants and many low-frequency ones. These high frequencies are thanks to the reproductive success of immigrants in the northern community and to the fruitfulness of socially dominant individuals in the central community. The southern community, however, has been declining, and they’ve been experiencing greater change in their Patr-B allele frequencies. While the northern and central communities have a SIVcpz prevalence of about 12 percent, Science reports, the southern group has a prevalence of 46.1 percent. 

One of the gene variants that’s elevated among SIVcpz-infected chimpanzees, called Patr-B*06:03, resembles the human MHC gene that delays the progression of HIV by reducing the viral load. It’s called HLA-B*57:01 and it triggers the production of killer cells that target HIV-infected cells in humans. Similarly, Patr-B*06:03 is linked to a reduced SIVcpz load, and it likely lessens the impact of SIVcpz infections as well. 

They’re not identical, but the chimp version is related to ours in structure, function, and evolution – forming a remarkable trans-species group of MHC alleles. Human and chimpanzee immune systems are remarkably similar, and understanding the differences and similarities between these MHC genes could help us to develop an effective vaccine.