CHAPTER 1 INTRODUCTION
1.4 HIV-1 Pathogenesis
1.4.4 Host immune responses and viral evasion
1.4.4.3 Cell-mediated immunity
1.4.4.3.1 CD8+ T cells
HIV-1-specifc CD8+ T cell responses have been detected as early as Fiebig stage I and II, however these responses appear to peak as viral load begins to decline [72] (Section 1.4.1).
The coincident peak of CD8+ T cell responses with viral decline suggests that they are an important contributor to the inhibition of virus replication in early infection [95]. This is further supported by experiments that demonstrated a marked increase in SI V viremia following depletion of CD8+ T cells in rhesus macaques [96, 97], while depletion of
24 antibody-generating B cells in an animal model had little impact on viral load decline in primary SIV infection [74, 98].
Cytotoxic CD8+ T cells recognise HIV-1 infected cells and subsequently eliminate them via the secretion of perforin and granzymes (reviewed in [91]). This recognition is dependent on the presentation of viral epitopes (typically 8-10 amino acid sequences produced by processing of viral proteins within the cytoplasm) by HLA class I molecules (encoded by the Major Histocompatibility Complex [MHC] locus) on the surface of infected cells to the CD8+ T cell. The CD8 molecule on the T cell interacts with a constant region on the HLA molecule, and assists binding of the T cell receptor (TCR) to the epitope-HLA complex.
This interaction is sufficient to trigger killing if the CD8+ T cell is already a differentiated effector cell, however naïve CD8+ T cells also require co-stimulatory signals (and this may be aided by CD4+ T cells) to become activated (Section 1.4.4.3.2). It should be noted that activated CD8+ T cells also produce antiviral cytokines (e.g. IFN-α) and chemokines (e.g.
MIP-1α and MIP-1β, which are CCR5 ligands), and recent evidence suggests that these non- cytolytic mechanisms of viral inhibition may in fact be more important than direct cell killing [99, 100].
The HLA class I molecule is a heterodimer of β2-microglobulin, which is a constant region shared by all class I proteins, and an α-chain, which is the variable region that recognises and non-covalently binds epitope [91]. Every individual expresses 6 HLA class I alleles (two HLA-A, two HLA-B, and two HLA-C alleles) and 6 HLA class II alleles (two HLA-DR, two HLA-DP, and two HLA-DQ alleles) (Section 1.4.4.3.2), of which there are currently 4,721 and 1,353 variant alleles, respectively, in total [101]. Each HLA molecule has a defined range of epitopes to which it can bind that is largely dependent on the epitope
residues that make direct contact with the HLA molecule (anchor residues) [91]. Thus, the HLA class I profile of an individual determines which viral epitopes mediate cytotoxic CD8+ T cell recognition and killing of infected cells, as well as other CD8+ T cell-mediated antiviral effects.
HIV-1 can counteract CD8+ T cell recognition through Nef-mediated selective downregulation of HLA class I molecules [36, 92]. Another important mechanism of evasion is the development of escape mutations in or adjacent to epitopes presented by the host HLA molecules. These mutations may impair intracellular processing, disrupt the binding of the epitope to the HLA molecule, or disrupt recognition of the presented epitope by the TCR [102-105]. CD8+ T cell escape mutations are rapidly selected as viremia declines, indicating that CD8+ T cell-mediated immune pressure makes an important contribution to this decline, while Env mutations selected by neutralising antibodies only appear approximately 12 weeks following infection [72, 106].
Mathematical modelling has estimated that 15-35% of HIV-infected cells are killed by a single CD8+ T cell response (of a few responses present) in early infection [72, 106] while only 4-6% of infected cells are eliminated by CD8+ T cells during chronic infection [107].
Therefore, the CD8+ T cells present in later infection are not as efficient as those in early infection, despite targeting a broader array of viral sequences (approximately 10 epitopes) [82, 108]. There may be several reasons for this impairment. The substantial loss of CD4+
T cells in acute infection results in decreased HIV-1-specific CD4+ T cell responses within the first few months and, therefore, a lack of CD4+ T cell help for subsequent CD8+ T cell responses [82]. CD4+ T cells are required for the maintenance of CD8+ T cell responses after acute infection [109], for their progression into long-term memory cells [72], and to
26 prevent rapid exhaustion [82]. Chronic high levels of HIV-1 antigen may also result in exhaustion, loss of function, and elimination of CD8+ T cells (Section 1.4.2). The resulting immune activation from HIV-1 infection may also impair CD8+ T cell responses by upregulating expansion of Fox P3+ CD25+ regulatory T cells, which inhibit the immune system by undetermined mechanisms that may include interleukin (IL)-10 and transforming growth factor-β secretion [82]. HIV-1 Nef might also aid in the evasion of CD8+ T cell responses through impairment of CD4+ T helper cell responses [36].
1.4.4.3.2 CD4+ T cells
HIV-1-specific CD4+ T cell responses are present in acute infection but decline rapidly [72, 82]. They provide help to innate immune cells, CD8+ T cells and B cells (reviewed in [91]).
Antigen presenting cells (APC; macrophages, dendritic cells, and B cells) engulf HIV-1 antigen into a phagolysosome where it is processed into peptides of 13-17 amino acids.
These peptides are then presented by HLA class II molecules on the APC surface to CD4+ T cells. The CD4 molecule on the T cell interacts with the HLA class II constant region and enhances binding of the TCR to the epitope-HLA complex. A co-stimulatory signal, principally the binding of B7 on the APC to CD28 on the CD4+ T cell, is required for activation of naïve CD4+ T cells. The activated CD4+ T helper 1 cells produce IL -2 which is required for the proliferation and differentiation of CD8+ T cells that have encountered antigen and other cytokines, such as IFN-γ, which promote the activity of macrophages. The CD4+ T helper 1 cells may also initiate co-stimulatory signals that are required for full activation of naïve CD8+ T cells encountering antigen. The activated CD4+ T helper 2 cells produce cytokines (e.g. IL-4 and IL-5) which stimulate proliferation of B cells that have encountered antigen and the production of soluble antibodies.
HIV-1 impairs all arms of the immune system through the depletion of CD4+ T helper cells (Section 1.4.2) with preferential infection and elimination of HIV-1-specific CD4+ T cells [110]. The depletion of uninfected CD4+ T cells is partly mediated by Nef-mediated upregulation of Fas ligand on infected cells that triggers apoptosis when binding to Fas on uninfected cells [1]. HIV-1 Nef further impairs CD4+ T cell function by inhibiting HLA class II epitope presentation and downregulating CD28 [36]. The preferential infection of the memory subset of CD4+ T cells also impairs immunity to previously encountered pathogens [111].
The general impairment of immune responses contributes to the gradual decline in immune function, the end result of which is death. Section 1.4 is summarised in Figure 1.2.