CHAPTER 1 INTRODUCTION
1.5 Therapy and preventative strategies
1.5.1 Antiretroviral therapy and prevention
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.
6 Figure 1.2 Pathogenesis of HIV-1 infection
(i) The graph shows viral load and CD4+ T cell count patterns, pathogenic events, approximate timing of initiation of immune responses, clinical manifestations and approximate time frames of the different stages of HIV-1 disease (acute phase, chronic phase, AIDS). Roman numerals indicate the Fiebig stages I-V of acute infection and the open-ended stage VI of early chronic infection. These Fiebig stages are characterised by the detection of HIV-1 RNA by PCR, p24 antigen by ELISA, and antibodies by Western blot or ELISA (shown below the graph). The graph is adapted from McMichael et al. (2010) [72]
and Kuritzkes and Walker (2007) [74].
(ii) The different host immune responses are depicted and some of the mechanisms by which HIV-1 evades, damages, and uses the host immune responses to its advantage are listed.
Dotted lines in immune response diagrams indicate binding, plus signs (+) indicate enhancement of activity and the asterisks (*) indicate the cells influenced by CD4+ T helper cells.
HIV-1 – human immunodeficiency virus type 1; AIDS – acquired immunodeficiency syndrome; RNA – ribonucleic acid;
CD – cluster of differentiation; PCR – polymerase chain reaction; p24 – protein of 24 kDa; Ab - antibody; p31 - protein of 31 kDa; E – enzyme-linked immunosorbent assay (ELISA); WB – Western blot; APC – antigen presenting cell; IFN-α – interferon alpha; NK cell – natural killer cell; KIR – killer cell immunoglobulin-like receptor; HLA – human leukocyte antigen; Ag - antigen; TCR – T cell receptor; Fc – fragment crystallisable; ADCC – antibody-dependent cell-mediated cytotoxic activity; Vpr – viral protein R; Nef – negative regulation factor; Vif – viral infectivity factor; Vpu – viral protein U; Env – envelope glycoprotein.
Downregulation of IFN-α (Vpr), impaired dendritic function, inflammation assists the establishment of infection
Pro-inflammatory environment facilitates establishment of infection, impaired phagocytosis, secretions promote resting T cell infection (Nef) Evade TRIM5α by capsid mutations, evade APOBEC3G by degradation (Vif), evade tetherin by sequestration (Vpu) Selective downregulation of HLA molecules (Nef)
Preferential infection of HIV-specific CD4+ memory T cells, triggering of apoptosis in uninfected cells (Nef), impaired antigen presentation by APC (Nef)
Downregulation of HLA (Nef), escape mutations, exhaustion and deletion of CD8+ T cells, impaired CD4+ T cell help
Germinal centre damage, abnormal B cell activation, production of autoantibodies, impaired CD4+ T cell help
Escape mutations, variable loops and carbohydrates masking conserved epitopes, germinal centre damage, impaired CD4+ T cell help
II
I III IV V VI
Eclipse
Acute Chronic AIDS
CD4+ T cell count (cells/mm3) 1000
750
500
250 6
5
4
3
2
1 Log10 HIV RNA copies/ml
10 20 30 101 1 2 3 4 5 6 7 8 9 10 11 12
Days post-infection Years post-infection
Dissemination reservoir established Latent reservoir
established reservoir established
Chronic immune activation and immune degradation reservoir established
symptoms
clinical latency
opportunistic infections
death
Undetectable RNA+ p24+ Ab+ Ab+/- Ab+ p31- Ab+ p31+
PCR E E WB WB E WB E
Host restriction factors Dendritic cell
Macrophage
NK cell
NK cell B cell
CD8 T cell
CD4 T cell infected
infected
infected
non- infected APC
APC
APC
upregulate pro-inflammatory
pro-inflammatory
+
+
+
+
perforin, granzymes KIR HLAI
Ag TCR
HLAII CD4
HLAI CD8
TCR Ag IFN-α
Fc Ab ADCC
neutralisation Block
1
2
3
4
5
6
7
8 Env Ab
Immune responses Immune evasion/degradation
*
*
+ +
+
*
*
1, 2, 3, 4, 5 6, 7 8
Infection reservoir established initiated
i
ii
antiviral activity
29 There are several challenges with ART. Even with highly potent therapy that suppresses viral load to undetectable levels there is low-level persistent residual HIV-1 replication from latently infected cells that become periodically activated, long-lived macrophages, and cells in anatomical sites such as the brain where penetration of drugs is limited [73, 78, 83, 112].
It is estimated that it would take 70 years of highly effective ART to purge this reservoir [74]. There are intensive efforts investigating the possibility of curing HIV-1 by activating latent reservoirs in combination with ART [83], and there is already an example of an HIV-1 cure in a man who received a bone marrow transplant from a CCR5 negative donor [114].
However, in the absence of a cure, complex treatment regimens that are difficult to maintain for long periods must be adhered to for life [115], with the high likelihood of drug resistance since greater than 95% adherence is required to avoid this. Continuous use of ART together with the persistent immune activation despite therapy increase the risk for cardiovascular disease, metabolic disorders, neurocognitive abnormalities, liver and renal disease, bone disorders, malignancy, and frailty [83]. Therefore, people on treatment do not reach a full life expectancy. In Denmark, an HIV-1 infected individual on therapy is half as likely to reach age 70 [116]. Furthermore, long-term ART for every infected individual for their lifetime is not sustainable due to high costs, and current coverage in low- and middle-income countries is only 40% [83].
Some novel ideas for therapeutics are being investigated, such as therapeutic vaccines and gene therapy. Two recent therapeutic vaccine trials of an adenovirus serotype 5 HIV-1 Gag vaccine and autologous dendritic cells pulsed with autologous heat-inactivated HIV-1 resulted in modest reductions in viral load and are therefore promising [117]. Delivery to mice of hematopoietic stem cells in which CCR5 alleles were disrupted (by a Zn finger nuclease) at a mean frequency of 17%, resulted in preferential expansion of CCR5 -negative
cells and HIV-1 control [118]. Thus, Zn finger nuclease treatment of patient hematopoietic stem cells is also a possible treatment approach.
Since delivery of ART is failing to keep pace with the number of infected individuals, several preventative approaches are being investigated to reduce the number of new HIV-1 transmissions. These include male circumcision, risk reduction counselling, condom use, and pre-exposure or post-exposure ART prophylaxis [119]. Recently 39% efficacy of protection from HIV-1 acquisition was shown in women using a 1% tenovifir gel before and after sexual intercourse [120]. However, the ideal way to combat the spread of HIV-1 is with an effective preventative vaccine. [94, 119, 121-123]. Historically vaccines have proven to be the most successful and cost-effective way to reduce the incidence of infectious diseases, such as polio, measles, mumps, rubella, hepatitis B, and influenza [94, 123]. The development of an effective HIV-1 vaccine has been elusive thus far.