The majority of CD4+ T-cell depletion during acute simian-human immunodeficiency virus SHIV89.6P infection occurs in uninfected cells

Abstract

Untreated human immunodeficiency virus (HIV) infection is characterized by depletion of CD4(+) T cells, ultimately leading to the impairment of host immune defenses and death. HIV-infected CD4(+) T cells die from direct virus-induced apoptosis and CD8 T-cell-mediated elimination, but a broader and more profound depletion occurs in uninfected CD4(+) T cells via multiple indirect effects of infection. We fit mathematical models to data from experiments that tested an HIV eradication strategy in which five macaques with a proportion of CD4(+) T cells resistant to simian-human immunodeficiency virus (SHIV) entry were challenged with SHIV89.6P, a highly pathogenic dual-tropic chimeric SIV-HIV viral strain that results in rapid loss of both SHIV-susceptible and SHIV-resistant CD4(+) T cells. Our results suggest that uninfected (bystander) cell death accounts for the majority of CD4(+) T-lymphocyte loss, with at least 60% and 99% of CD4(+) T cell death occurring in uninfected cells during acute and established infection, respectively. Mechanisms to limit the profound indirect killing effects associated with HIV infection may be associated with immune preservation and improved long-term survival.

Importance: HIV infection induces a massive depletion of CD4(+) T cells, leading to profound immunodeficiency, opportunistic infections, and eventually death. While HIV induces apoptosis (programmed cell death) by directly entering and replicating in CD4(+) T cells, uninfected CD4(+) T cells also undergo apoptosis due to ongoing toxic inflammation in the region of infection. In this paper, we use mathematical models in conjunction with data from simian-human immunodeficiency virus SHIV89.6P infection in macaques (a model of HIV infection in humans) to estimate the percentage of cell death that occurs in uninfected cells during the initial period of infection. We reveal that the vast majority of cell death occurs in these cells, which are not infected. The "bystander effects" that lead to enormous reductions in the number of uninfected CD4(+) T cells may be a target for future interventions that aim to limit the extent of damage caused by HIV.

FIG 1

(A) SHIV RNA trajectories in plasma for the five macaques. (B) Total CD4 T-cell trajectories in plasma for the five macaques. (C to E) Unmodified and modified populations of CD4⁺ T cells for the three experimental animals (E1 to E3). (C) E1 had 10% modified cells prior to infection; (D) E2 had 17% modified cells prior to infection; (E) E3 had 55% modified cells prior to infection. This research was originally published in Blood (2) and is reprinted here with permission from the publisher, the American Society of Hematology.

FIG 2

mC46 expression potently inhibits infection of primary cells both in vivo and in vitro. (A) Primary CD4⁺ T cells, macrophages, and monocyte-derived dendritic cells (MDDCs) were incubated for 4 h with SHIV89.6 Env-pseudotyped, replication-deficient reporter virus encoding luciferase (2 ng p24). Cells were washed and cultured for an additional 3 days in appropriate medium for each cell type. Cells were lysed in passive lysis buffer, and luciferase activity was detected using a luminometer. A minimal increase was observed in mC46-expressing CD4⁺ T cells (1.1-fold), macrophages (1.09-fold), and MDDCs (1.12-fold). (B) The proviral DNA copy number was determined following sorting of GFP⁺ and GFP⁻ CD4⁺ T cells isolated from peripheral blood of SHIV89.6-infected macaques. Values were adjusted to the number of proviral DNA copies per 10⁶ CD4⁺ T cells. This research was originally published in Blood (2) and is reprinted here with permission from the publisher, the American Society of Hematology.

FIG 3

Best-fit trajectories and experimentally observed data for the experimental and control animals. The indirect effects model (model 2; see the Appendix) was fit with 7 unknown parameters for experimental animal 1 (E1). All parameter values were then carried forward except for a single parameter (the proportion of SHIV produced by CD4⁺ T cells in blood (p). This unique parameter was fit to the data of the other 4 animals. (A to C) Data for E1, which had 10% modified cells prior to infection. (D to F) Data for E2, which had 17% modified cells prior to infection. (G to I) Data for E3, which had 55% modified cells prior to infection. (J to K) Data for control animal 1 (C1). (L to M) Data for C2.

FIG 4

(A) Modeled viral loads for the five macaques. (B) Modeled total CD4 T-cell counts for the five macaques. (C) Modeled modified CD4⁺ T-cell counts for the experimental macaques. The indirect effects model (model 2; see the Appendix) was fit with 7 unknown parameters for experimental animal 1. All parameter values were then carried forward except for a single parameter (the proportion of SHIV produced by CD4⁺ T cells in blood, p). This unique parameter was fit to the data for the other 4 animals.

FIG 5

Kinetics of cell death rates during SHIV infection (the predictive model fits were used for these plots). (A) Indirect (solid lines) versus direct (dashed lines) death rates as functions of time. The indirect cell death rate encompasses the indirect terms affecting all target cells in the model (susceptible, modified, infected, and modified infected), while the direct effect rate accounts for the rate at which infected cells die from direct virus-induced apoptosis in the model. (B) Percentage of total cell death due to indirect effects (solid lines) and to direct virus-induced apoptosis of infected cells (dashed lines) during infection. (C) Percentage of total cell death in uninfected (bystander) cells (solid lines) versus infected (dashed) lines. (D) Rates of indirect (solid) versus direct (dashed) removal per cell for susceptible cells.