IRON AND HIV INFECTION
During HIV infection alterations occur in iron distribution that are mediated through changes in the primary iron and haem transporters and storage molecules (transferrina, ferritina, lactoferrina, Hp, haemopexin and caeruloplasmine) and recuction of intestinal iron absorption regulated by hepcidina: this is a defensive strategy of the APR (acute-phase-response) used by our organism to reduce the disponibility of iron for patogens.
On the other side this lands to an unintended consequence wich is anemia of chronic disease (ACD) in which citokine blockade of iron distribution can yeld the co-existance of peripheral anemia and iron overload in the liver and/or bone marrow. Therefore, HIV-1 infection can cause paradoxical alterations in iron metabolism with the co-existence of anaemia alongside high circulating ferritin and iron deposits suggestive of iron overload.
Many of the studies reporting abnormal iron status have been conducted in individuals with advanced HIV infection, it is difficult to determine the temporal sequence of these alterations which might be due to HIV infection itself, to opportunistic infections and their complications or to treatments that influence iron status in HIV infection. Altered iron metabolism in HIV infection may present as hypoferraemia, hypotransferrinaemia, low or high Tf saturation and moderate to severe hyperferritinaemia combined with increased deposition of iron ferritin and/or haemosiderin in macrophages, microglia, Kupffer cells, endothelial cells and myocytes of the bone marrow, brain white matter, skeletal muscle and sometimes liver. Ref
Clinical studies have frequently observed macrophage iron loading in patients who are HIV-1 positive, and one such investigation demonstrated a positive correlation between the degree of iron loading in bone-marrow macrophage and mortality.There are several reasons why the accomulation of iron in macrophages might be an indicator of poor outcome in Hiv infection.
1- Macrophages harbour an important reservoir of HIV-1 that is comparatively drug resistant.
2- Certain steps in the HIV-1 replicative cycle involve iron (see figure):
- After viral binding to CD4 and its co-receptor, fusion and cell entry, viral is reverse transcribed into DNA through a proces that requires the iron-dependent protein Ribonucleotide Reductase.
- Nuclear factor (NF)-kB initiates transcription of the integrated viral genome from the 5'LTR (long terminal repeat) and it is the iron to activate NF-kB though generating Reactive Oxygen Speaces and activate a Kinase that leads to the NF-kB localization into the nucleus.
- The transcription of the HIV genome needs the HIV protein Tat that it is inhibited in condition of low cellular iron.
- Unspliced and partially spliced viral RNAs need to be exported from the nucleus before theyr encoded genes can be expressed. This export needs a host protein complex in which takes part the aminoacid HYPUSINE (HY), derived by a post translation modification of Lysine carried out by iron depending enzymes. Inhibiting HY sintesis block HIV-1 replication.
- Even assembly of the GAG capside proteins into mature virions requires an iron-binding ATPases: ABCE1, a protein essential for translation initiation and ribosome biogenesis in our cells, but recruited by HIV-1 Gag and co-opted in virion construction.Seems obvious that iron could promote HIV-1 replication.
3- In vivo macrophage iron loading in the contest of HIV-1 infection could have further consequences that would be detrimental to the host: infections by secondary pathogens (charateristic of AIDS) becouse these organism requires iron to grow and some as M.Tuberculosis target macrophaces. Therefore iron accomulations in macrophages might favour not only HIV-1 replication but also colonization with other patogenic microorganisms.
4- Cells that produce most HIV-1 virions can be co-infected by secondary pathogens. For example also EBV can enhance Linfo B capacity of incorporate iron: cells coinvected by HIV and EBV can easier lead to Linfoma.
5- Of course, another detrimental side effect of chronic iron deposition in macrophages is anemia, affecting erytropoiesis. Such anemia of chronic diseas is observed in various viral infections and unsurprisingly is associated with a poor prognosis.
How can macrophages fill-up with iron?
Macrophages take up iron from the haemoglobine of red blood cells as itself or complexed with Haptoglobin. This uptake is mediated by the macrophage scavenger receptor CD163, wich has ten fold higher affinity for the Haptoglobine phenotipic variant Hp2.2.
The Haptoglob2.2 variant has been linked with significantly higher mortality in HIV infection.
Expression of CD163 can be alterated in patients vith HIV-1
Once internalized iron is liberated from haemoglobin by haem oxygenase, and the transporters DMT1 and solute carrier Slc11a1 mediate iron transfer between the lysosome and the cytosol. Slc11a1 influence susceptibility to M.tuberculosis and other patogens and has also been implicated in NF-kb activation, wich induces HIV-1 transcription. Both Aptoglobina and Slc11a1 (NRAMP1) polymorphisms could therefore influence HIV/AIDS. Ref
The final stage of iron recycling is its export from the macrophage by Ferroportin, a process that is inhibited by Hepcidin. In some cases of anemia of chronic disease, an enhancement of hepcidin levels that in brought about by inflammation is thought to be responsible for the accumilation of iron in macrophages and the suppression of eritropoiesis.
Also it is known that HIV-1 Nef protein causes a reduction of the numbers of cell surface MHC class I proteins making the infected cell less visible to the virus specific T cells. Nef can also affect HFE localization and the rerouting og HFE by Nef from the plasma membrane to the trans-Golgi networks is accompanied by accumulation of iron in macrophages. The action of Nef on HFE suggest that HIV-1 can engineer an iron rich environment in macrophages that benefits the virus but harms the host. Ref
In vitro this iron accumulation does not occur when macrophages from hemocromatosis patients with disabling HFE mutation are infected. Wether HFE mutations influence HIV-1 progression in vivo, perhaps by counteracting macrophage iron deposition, is not certain. However, HIV infected long term non progressor with hemochromatosia has been described. Ref
Consequences of HIV-associated anaemia Anaemia is the most commonly reported haematological disturbance in HIV infection with prevalence estimates ranging from 10–95 % in different clinical settings and populations. Epidemiological studies of HIV- related anaemia have demonstrated repeatedly strong associations between low haemoglobin concentrations and disease progression and mortality. Ref
Those at greater risk for anaemia are of African–American descent, with low CD4 + count, high viral load, low mean corpuscular volume and those receiving AZT. Ref
Although the association between anaemia and decreased survival has not been proven to be causal, the consistent findings of numerous studies of large observational cohort design suggest anaemia is an important factor in understanding the clinical course of subjects with HIV.
Investigating the putuative links between iron status, disease progression and mortality in patients infected with HIV-1 is complicated by some confounding conditions: most measures of iron status are strongly influenced by the APR, which is frequently active in individuals infected with HIV-1; APR related redistribution of iron can result in paradoxical iron depletion in some body compartments and iron overload in others. Noumerous studyes confirm that anemia leads to a poor prognosis.
A study from 2006 demonstrated that higher indirect measures of iron status (serum ferritine and transferrie receptor concentrations) were associated with reduced survival among HAART-naive HIV-infected women. Additional prospective studies with data on direct measures of iron status along with randomized trials are needed to elucidate the current equipoise over whether iron supplementation is beneficial by preventing anemia or harmful by increasing iron stores in HIV-infected women.Clinical evidence for associacion between iron status and HIV-1 progression
Not only is anaemia associated with a poorer prognosis, but haematological recovery is associated with an improved prognosis and better quality of life. Treatment of anaemia was associated with improved survival times over a range of CD4 counts; however, some data suggest that caution is warranted in the management of anaemia. Iron suplementation it is used in the intent to reduce mortality in HIV infected children: Ref
Blood transfusions were problematic and associated with accelerated mortality, even early in the course of HIV infection. Ref
This could be secondary to transfusion-transmitted infection, transfusion-related immunosuppression (an immunomodulatory effect of up-regulating humoral immunity and down-regulating cell-mediated immunity)or possible transient activation of HIV expression, which may provide an opportunity for developing viral resistance if concurrently using antiretrovirals. A study demonstrated that ferritin was negatively correlated with haemoglobin and positively correlated with neopterin , a marker of both macrophage activation and HIV disease progression: Ref
On the other side, increased haemoglobine levels by antiretroviral therapy is associated with a decrease of immune attivation:
Increasing concentrations of ferritin were accompanied, or closely preceded, by rapid disease progression in HIV-seropositive children. Other studies have shown that serum ferritin concentrations in HIV- seropositive subjects were associated with the severity of infection when evaluated clinically as disease stage, immunologically as CD4 + cell count or virologically as serum p24 antigen title and plasma viral burden.Given these early indications of a role for elevated iron in vivo, mechanisms that might explain the possible detrimental effect of iron have been subsequently investigated in HIV infection. These include: direct cytotoxicity and immune dysfunction; enhancement of viral replication; predisposition to certain opportunistic infections and neoplasia; and alterations in the immune response that may affect HIV virulence. Iron loading is associated with the active production of hydroxyl radicals through the Haber–Weiss or Fenton reaction. The resulting ROS (reactive oxygen species) are associated with the oxidation of nucleic acids, chromosome breaks and peroxidation of unsaturated fatty acids.
It is possible for limited chromosomal damage to be repaired; however, extensive DNA damage promotes apoptosis of the affected cell. A number of studies have shown that ROS may play a role in HIV progression by induction of apoptosis, with the ensuing depletion of CD4 + cells. Antioxidants may inhibit apoptosis in vitro and several little studyes was demonstrated this effect in vivo. Ref
Chronic oxidative stress has also been linked hypothetically with abnormal immune function, particularly T-cell function. In vitro findings suggest the involvement of ROS in signal transduction pathways leading to the activation of NF-κ B (nuclear factor κ B) which potently increases the production of HIV virons in latently infected cells. Ref
Given that ROS probably promotes HIV transcription, it is important to consider if antioxidants are associated with a reduction in HIV viral replication, or conversely, if iron supplementation is associated with enhanced viral load. A number of studies in the early 1990s have shown a reduction in virus replication associated with antioxidant supplementation in vitro (primarily using non-nutrient antioxidants, with the exception of ascorbate); however, human studies have not been able to consistently replicate these effects.Very limited research has been published examining the relationship between iron supplementation and HIV viral load. A recent report of a randomized placebo-controlled double-blind clinical trial from Kenya did not show enhanced HIV-1 viral load in subjects receiving iron supplements (60 mg of elemental iron, twice weekly, for 4 months): Ref
This dosage, however, is below the standard daily dose for treatment of anaemia and anaemia prevention in pregnant women and, thus, further investigation is required. The direct effect of iron as a growth factor for specific pathogens causing opportunistic infections in HIV has been investigated, and it has been shown in vitro and in murine models that iron influences the development of Mycobacterium infections.If iron excess is associated with a negative outcome in HIV infection, is iron chelation associated with an improved prognosis? Chelatable iron or ‘free’ iron is a labile iron compartment in dynamic equilibrium with free iron stored in tissues. Ref
Iron chelation has been shown to have an effect in vitro on HIV-1 replication. When two HIV-1 provirally infected cell lines were exposed to oxidant stress induced by H2 O2 , DFX inhibited the activation of NF-κ B. This resulted in a significant reduction in p24 antigen production and reverse transcriptase activity in the absence of cytotoxicity. A similar antiretroviral effect was demonstrated using HIV-1-infected peripheral blood mononuclear cells stimulated with IL (interleukin)-2. Ref
DFX, as well as several antioxidants, have been shown by others to block the induction of NF-κ B by oxidant stress, and DFX has been shown to protect against TNFα (tumour necrosis factor α)- mediated cytotoxicity and HIV-1 replication in other cell lines. A second route by which iron chelation could influence HIV replication is by inhibition of DNA synthesis through inactivation of ribonucleotide reductase. Ribonucleotide reductase inhibition by DFX has been reported and this was accompanied by inhibition of lymphocyte proliferation. Ref
It was also observed that the expression of TfRs (Tf receptors) is down-modulated by acute HIV-1 infection in T-lymphoid cells and thus cell phenotypic modulation is associated with the cytopathic effects of the virus and, importantly, this effect can be modulated by iron chelation (with sodium citrate or DFX).
Ref, an association between iron metabolism and CQ (chloroquine) with anti-HIV activity has been demonstrated in a number of studies. Ref