|August 7, 2012|
HIV-1-Infected Humanized Mice Mirror Human HIV-1 Infection
Arms races don’t only occur among warring nations. They also occur on a microscopic scale, and just as insidiously, when our immune system battles particularly nasty pathogens like HIV-1. Despite our immune system’s attempts to eliminate it, HIV-1 hides, mutates, and mounts counterattacks. In response, our immune system adapts, unveils another weapon from its arsenal, and mounts its own counterattack. On and on it goes, and, unfortunately, HIV-1 usually wins. To win the arms race with HIV-1, we need to better understand the interactions between it and our immune system. Although many studies have heralded the ability of humanized mice to be reconstituted with a human immune system, few have demonstrated that the interactions between the humanized immune system and pathogens in these mice accurately mimic what happens in humans. The laboratory of Dr. Todd Allen of the Ragon Institute of Massachusetts General Hospital, Massachusetts Institute of Technology, and Harvard, Charlestown, MA, recently published such a study. Allen and his colleagues found that HIV-1-infected humanized NOD.CB17-Prkdcscid/J (NOD/Scid, 001303) and NOD.Cg-Prkdcscid Il2rgtm1Wjl/SzJ (NSG, 005557) BLT mice accurately mirror human cellular immune responses to HIV-1 infection and HIV-1’s mutations to escape those responses (Dudek et al. 2012). Allen’s findings indicate that humanized mice fundamentally advance our ability to accurately model the immunology of human infectious disease pathogenesis and to identify targets and develop therapies for those diseases.
CD8+ T cells and HIV-1
CD8+ T cells, also known as cytotoxic T lymphocytes or killter T cells, are white blood cells that specialize in killing cancer cells and cells infected by various pathogens, especially viruses. They play a seminal role in attacking HIV-1 in the initial acute stages of infection. Allen and his colleagues wanted to know if HIV-1-infected humanized BLT mice accurately model the dynamic interactions between CD8+ T cells and HIV-1 in HIV-1-infected humans.
Producing BLT mice
To produce humanized BLT (bone marrow-liver-thymus) mice, Allen and his team sub-lethally irradiated NOD/Scid and NSG mice, transplanted each with liver and thymus tissue from different human donors (each expressing distinct human leukocyte antigen (HLA haplotypes), and engrafted them with CD34+ hematopoietic stem cells (HSCs). In 12-20 weeks, these mice became reconstituted with human immune cells (NOD/Scid mice reconstituted in 16-20 weeks; NSG mice reconstituted in as little as 12-14 weeks).
|To produce humanized BLT mice, Dr. Allen used NOD.CB17-Prkdcscid/J (NOD/Scid, 001303) (left) and NOD.Cg-Prkdcscid Il2rgtm1Wjl/SzJ (NSG, 005557) (right) mice.|
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HIV-1/immune responses in humanized BLT mice mirror those in humans
Allen and his colleagues infected the humanized BLT mice with the HIV-1 JR-CSF molecular clone and analyzed the CD8+ T cell and HIV-1 responses over time. Their key findings are summarized below:
Two key epitopes are involved
Between six and twelve weeks post-infection, HIV-1 viral sequence evolves (mutates) in a clear and mostly nonsynonymous pattern in two commonly targeted CD8+ T cell epitopes. Correspondingly, early mutations within the CD8+ T cells of BLT mice arise disproportionately within the same two epitopes. As in HIV-1-infected humans, HIV-1 “escape” mutations within these two epitopes are rapid and highly reproducible, suggesting that humanized BLT mice develop multiple normal (human) CD8+ T cell responses to HIV-1. The fact that HIV-1 escape mutations arise primarily in two CD8+ T cell epitopes suggests that these epitopes are under strong immune selection pressures. Mutations in these epitopes can completely impair the ability of CD8+ T cells to recognize HIV-1, demonstrating how ongoing sequence evolution of the viral genome evades the hosts immune system.
HIV-1’s mutations are HLA-restricted
Similar HIV-1 escape mutations arise among BLT mice humanized with tissues from human donors expressing the same HLA haplotypes, demonstrating that HIV-1’s escape mutations in humanized BLT mice are not random but restricted by the host’s HLA allele.
CD8+ T cells control HIV-1 via APOBEC3G/F
As in humans, early HIV-1 escape mutations in humanized BLT mice are stochastic, and the CD4+ T cells in these mice inhibit early viral replication through the cytidine deaminase hypermutation effects of the host factor APOBEC3G/F.
The HLA-B*57 allele confers better HIV-1 control
As with humans, humanized BLT mice that express the HLA-B*57 allele can better control HIV-1 replication by targeting the highly conserved HIV-1 Gag (Freed 1998) CD8+ T cell epitope. Although HIV-1 mutations in humanized BLT mice arise reproducibly and rapidly in primarily two CD8+ T cell epitopes, very few or none arise in the Gag epitopes.
In summary, the Allen team demonstrated that HIV-1 infection in humanized BLT mice accurately models human immune and viral responses. The CD8+ T cell responses to HIV-1 and the spontaneous mutations of HIV-1 to escape those responses in humanized BLT mice that express the same HLA alleles are reproducible. This suggests that humanized BLT mice with HLA-matched immune systems can be used to study human CD8+ T cell-HIV-1 interactions and to develop new HIV/AIDS targets and therapies.