Recapitulating Cross-Species Transmission of Simian Immunodeficiency Virus SIVcpz to Humans by Using Humanized BLT Mice

Abstract

The origins of human immunodeficiency virus type 1 (HIV-1) have been widely accepted to be the consequences of simian immunodeficiency viruses from wild chimpanzees (SIVcpz) crossing over to humans. However, there has not been any in vivo study of SIVcpz infection of humans. Also, it remains largely unknown why only specific SIVcpz strains have achieved cross-species transmission and what transmission risk might exist for those SIVcpz strains that have not been found to infect humans. Closing this knowledge gap is essential for better understanding cross-species transmission and predicting the likelihood of additional cross-species transmissions of SIV into humans. Here we show that humanized bone marrow, thymus, and liver (hu-BLT) mice are susceptible to all studied strains of SIVcpz, including the inferred ancestral viruses of pandemic and nonpandemic HIV-1 groups M (SIVcpzMB897) and N (SIVcpzEK505) as well as strains that have not been found in humans (SIVcpzMT145 and SIVcpzBF1167). Importantly, the ability of SIVcpz to cross the interspecies barrier to infect humanized mice correlates with their phylogenetic distance to pandemic HIV-1. We also identified mutations of SIVcpzMB897 (Env G411R and G413R) and SIVcpzBF1167 (Env H280Q and Q380R) at 14 weeks postinoculation. Together, our results have recapitulated the events of SIVcpz cross-species transmission to humans and identified mutations that occurred during the first 16 weeks of infection, providing in vivo experimental evidence that the origins of HIV-1 are the consequence of SIVcpz crossing over to humans. This study also revealed that SIVcpz viruses whose inferred descendants have not been found in humans still have the potential to cause an HIV-1-like zoonosis.

Importance: It is believed that the origins of HIV-1 are the consequence of SIV from wild chimpanzees crossing over to humans. However, the origins of HIV-1 have been linked back to only specific SIVcpz strains. There have been no experiments that directly test the in vivo cross-species transmissibility of SIVcpz strains to humans. This is the first in vivo study of SIVcpz cross-species transmission. With the humanized-BLT mouse model, we have provided in vivo experimental evidence of multiple SIVcpz strains crossing over to humans and identified several important mutations of divergent SIVcpz strains after long-term replication in human cells. We also found that the cross-species transmission barrier of SIVcpz to humans correlates with their phylogenetic distance to pandemic HIV-1 group M. Importantly, this work provides evidence that SIVcpz viruses, whose inferred descendants have not been found in humans, still have the potential to cause a future HIV-1-like zoonotic outbreak.

FIG 1

The phylogeny of SIVcpz, SIVgor, and HIV-1. The evolutionary relationship of SIVcpz, SIVgor, and HIV-1 groups M, N, O, and P is based on pol sequences, whose coordinates are 3887 to 4778 on the HIV-1/HXB2 genome. The viruses used in this study are highlighted in boxes, and virus host species are indicated on the right. The scale bar represents 0.06 amino acid replacements per site. G. g. gorilla, Gorilla gorilla subsp. gorilla.

FIG 2

SIVcpz pVL kinetics. (A to E) Plasma VL over the course of up to 16 wpi in 5 groups of hu-BLT mice inoculated with a high dose of each virus: SIVcpzMB897 (A), SIVcpzEK505 (B), SIVcpzMT145 (C), SIVcpzBF1167 (D), and HIV-1_SUMA (E). (F) Plasma VL kinetics of all 5 groups based on the mean values. The dashed line indicates the detection limit of pVL.

FIG 3

SIVcpz RNA⁺ cells and infected cell types in lymphoid tissues. Representative images of lymph node tissues of hu-BLT mice at 16 wpi detected using in situ hybridization (ISH) with ³⁵S-labeled probes (A and B) and a combination of ISH and IHC staining (C to F). Viral RNA⁺ cells in SIVcpzEK505-infected hu-BLT mouse detected with antisense probe (A) or with sense probe as a negative control (B). The majority of viral RNA⁺ cells are colocalized with CD4⁺ T cells (C and D, blue arrows), and some of the viral RNA⁺ cells are colocalized with macrophages (E and F) in SIVcpzBF1167-infected hu-BLT mouse. Viral RNA⁺ cells that are not colocalized with macrophages are indicated with red arrows (F).

FIG 4

Cross-species transmission barrier of SIVcpz. Five groups of hu-BLT mice were inoculated with a low dose of SIVcpz or HIV-1_SUMA normalized through RT activity (n = 5 or 6). The Kaplan-Meier plots for conversion to infected status are in red for HIV-1_SUMA (A through D), in dark blue for SIVcpzMB897 (A), in light blue for SIVcpzEK505 (B), in dark green for SIVcpzMT145 (C), and in light green for SIVcpzBF1167 (D). The P value indicates the significance of differences between the two curves, and P values of <0.05 are considered significant.

FIG 5

Env sequencing revealed the mutation of SIVcpz in vivo at 14 wpi. The coverage and depth (square brackets) of next-generation sequencing of SIVcpzMB897 and SIVcpzBF1167 are shown. The expanded images highlight the mutation positions and ratios: A is in green, T in red, G in orange, and C in blue. (A) SIVcpzMB897 cross-animal mutations at positions 1231 and 1237 in hu-BLT mice 273 and 308. (B) SIVcpzBF1167 cross-animal mutations at positions 840 and 1129 in hu-BLT mice 480, 482, and 483. (C) Amino acid mutations mapped to the corresponding positions on HIV-1 Env trimer. GP120 is in light brown, GP41 is in light gray, the CD4 binding loop is in yellow, SIVcpzMB897 G411R and G413R are in purple, SIVcpzBF1167 H280Q is in green, and Q380R is in red.

FIG 6

Sequence logos for the identified Env AA mutations. For the SIVcpz BF1167 H280Q mutation, Q is absent in SIVcpz but present in HIV-1 with low frequency; similarly, for mutation Q380R, R is absent in SIVcpz but present in HIV-1 with low frequency. For the MB897 G411R mutation, R is present in both SIVcpz and HIV-1; and for the MB897 G413R mutation, R is absent in SIVcpz but present in HIV-1. Star, original AA in the SIVcpz viruses; arrow, mutated AA in HIV-1 at their corresponding positions.

FIG 7

Gag M30R mutant and wild-type in vivo competition at 4 wpi. Red circle, position of Gag 30. (A) SIVcpzMB897 in vivo equal-copy-number competition. (B) SIVcpzMB897 in vivo equal-IU competition. The red line highlights the only animal with opposite selection. (C) SIVcpzEK505 in vivo equal-copy-number competition. (D) SIVcpzEK505 in vivo equal-IU competition.