Natural Killer Cells in Antibody Independent and Antibody Dependent HIV Control

Abstract

Infection with the human immunodeficiency virus (HIV), when left untreated, typically leads to disease progression towards acquired immunodeficiency syndrome. Some people living with HIV (PLWH) control their virus to levels below the limit of detection of standard viral load assays, without treatment. As such, they represent examples of a functional HIV cure. These individuals, called Elite Controllers (ECs), are rare, making up <1% of PLWH. Genome wide association studies mapped genes in the major histocompatibility complex (MHC) class I region as important in HIV control. ECs have potent virus specific CD8⁺ T cell responses often restricted by protective MHC class I antigens. Natural Killer (NK) cells are innate immune cells whose activation state depends on the integration of activating and inhibitory signals arising from cell surface receptors interacting with their ligands on neighboring cells. Inhibitory NK cell receptors also use a subset of MHC class I antigens as ligands. This interaction educates NK cells, priming them to respond to HIV infected cell with reduced MHC class I antigen expression levels. NK cells can also be activated through the crosslinking of the activating NK cell receptor, CD16, which binds the fragment crystallizable portion of immunoglobulin G. This mode of activation confers NK cells with specificity to HIV infected cells when the antigen binding portion of CD16 bound immunoglobulin G recognizes HIV Envelope on infected cells. Here, we review the role of NK cells in antibody independent and antibody dependent HIV control.

Keywords: ADCC; HIV; HLA; NK cells; antibody dependent NK cell activation; elite controllers; killer immunoglobulin-like receptors.

Figure 1

Killer Immunoglobulin-like Receptor Haplotypes. KIR genes are organized into haplotypes A and B. KIR haplotype A is comprised of four framework genes present in most KIR haplotypes (KIR3DL3 at the centromeric end, KIR3DL2 at the telomeric end and the pseudogene KIR3DP1 and KIR2DL4 in the middle) plus genes encoding inhibitory KIRs KIR2DL1, KIR2DL3, KIR3DL1, activating KIR KIR2DS4 and pseudogene KIR2DP1. The more diverse group B haplotypes include the framework genes with various combinations of genes encoding inhibitory KIRs KIR2DL2, KIR2DL5A/B and activating KIRs KIR2DS1, KIR2DS2, KIR2DS3, KIR2DS5 and KIR3DS1. Most KIR region haplotypes are composed of one of 3 centromeric and one of 3 telomeric KIR motifs that include combinations of KIR genes in linkage disequilibrium with each other, which are usually inherited together. The hotspot in the center between the centromeric and telomeric regions allows for frequent recombination between the two regions. The centromeric region is delimited by the framework genes KIR3DL3 and KIR3DP1 while the telomeric region is delimited by framework genes KIR2DL4 and KIR3DL2. KIR2DP1 and KIR2DP1 are pseudogenes. CA01 = genes in the centromeric region of KIR haplotype A 1; CB01 = genes in the centromeric region of KIR haplotype B 1; CB02 = genes in the centromeric region of KIR haplotype B 2; TA01 = genes in the telomeric region of KIR haplotype A 1; TA02 = genes in the telomeric region of KIR haplotype A 2; TB01 genes in the telomeric region of KIR haplotype B 1.

Figure 2

Antibody independent NK cell anti-HIV activity. (1) HIV-uninfected CD4⁺ cells express major histocompatibility complex (MHC) class I antigens (i.e. HLA-B*57/*27). These MHC class I ligands can interact with the inhibitory Killer Immunoglobulin-like Receptor (KIR) KIR3DL1. (A) When this interaction occurs during NK cell ontogeny, NK cells are educated to be tolerant to normal self-cells. (B) In HIV infected CD4 cells, Nef (orange circle) downregulates MHC I (i.e. HLA-B*57/*27), which are ligands for the inhibitory KIR3DL1 (1). Absence of the cell surface ligands for KIR3DL1 abrogates negative signaling through this receptor changing the balance between negative and positive signals received towards NK cell activation (2) and release of inflammatory cytokines such as tumor necrosis factor α (TNF-α) and interferon γ (IFN-γ) and chemokines such as CCL3, CCL4 and CCL5 (3). CCL4 (and CCL3 and CCL5) bind to CCR5, the co-receptor for HIV entry (4), which blocks the infection of uninfected, bystander CD4⁺ cells (5).

Figure 3

Comparison of ADCC using target cells coated with HIV Envelope gp120 versus Envelope on HIV infected cells. (A) HIV Envelope gp120-CD4 interactions produce an open Envelope conformation exposing CD4 induced (CD4i) epitopes that are hidden in trimeric closed conformation HIV Envelope. This occurs when target cells are gp120-coated cells or bystander CD4⁺ cells binding gp120 shed from co-cultured HIV infected cells. This conformation can be recognized by antibodies to CD4i epitopes (shown in blue), some broadly neutralizing antibodies (BnAbs, shown in black) but not by most BnAbs (shown in red). In gp120-coated cells, the HIV proteins Envelope (green), Nef (orange circle) and Vpu (light blue circle) are absent. The Fc portion of antibodies to the CD4i epitope bind the activating NK cell receptor, CD16/FcR, which activates NK cells to secrete cytokine, chemokines and lytic granules to kill target cells by ADCC. (B) Nef and Vpu encoded by HIV downmodulate CD4 preventing CD4 from interacting with HIV Envelope. The Envelope is thus presented on the surface of HIV infected cells is its closed conformation. BnAs shown in red and black recognize this closed Envelope conformation while antibodies to the CD4i epitope shown in blue do not. The Fc portion of Envelope bound antibodies binds CD16/FcR to activate NK cells to secrete cytokine, chemokines and lytic granules to kill target cells by ADCC.

Figure 4

ADNKA and ADCC activity. Nef- and Vpu-mediated downregulation of cell surface CD4 prevents the interaction of CD4 with HIV Envelope leaving it in a closed trimeric conformation on the HIV infected cell surface. The antigen combining site of BnAbs (shown in red) bind this Envelope conformation while the Fc region of the BnAbs bind the activating receptor CD16/FcR on NK cells. ADNKA assesses the secretion of inflammatory cytokines such as TNF-α (green) and IFN-γ (red), and expression of the degranulation marker CD107a (purple) from NK effector cells (left) whereas ADCC assesses the cytopathic effects of cytolytic granules containing perforin and granzyme B released by NK cells on antibody opsonized target cells (right).