The course of HIV-1 infection

1.5.1 HIV-1 transmission and establishing infection

Different routes of transmission come with a different rate of a successful HIV-1 infection. Heterosexual transmission accounts for roughly 70% of HIV-1 infections.

The risk for a man to be infected by having intercourse with an infected woman is 1 in 700 to 1 in 3000, while it is more likely for a women to get infected by having intercourse with an infected male (1 in 200 to 1 on 2000). The remaining infections are accounted for mostly by men who have sex with men (MSM), maternal-infant infections and injection drug use. The risk of rectal transmission is considerable higher with 1 in 20 to 1 in 300. While the contact with infected blood during birth bears a 1 in 10 to 1 in 20 risk, injection drug use or accidents with HIV-1 positive blood can lead to transmission in 1 in 150 events [79]. Drug injectors and MSM have 22-fold and 34-fold higher risks to be infected with HIV-1, respectively (UNAIDS Report 2012). The lowered risk for heterosexual transmission can be attributed to the fact that the genital mucosa serves as a first physical barrier to the virus and creates a bottleneck for the initial transmitted viral population (Figure 1.3).

In fact sequence analysis and mathematical modeling of viral genomes of patients showed that in most cases only one virus/genome is establishing the systemic infection [80]. The virus crosses the genital epithelium in about 30 to 60 minutes as demonstrated in an SIV macaque model [81]. It is not clear if the virus passes the epithelial layer through the cells via endocytosis and subsequent exocytosis with or without productive infection or traverses through gaps in the epithelium and mechanical micro-abrasions in the layer occurring during intercourse. It has been shown in mice, macaques and human studies that - next to cell free virus - donor leukocyte associated virus can be the source of infection ([82-84] and reviewed here [79, 85]).

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The mucosa is lined with tissue specific dendritic cells, Langerhans cells (LCs), which belong to the subtype of conventional dendritic cells (cDC), their dendrites can extend through the epithelial layer and capture viral particle [86, 87]. The LCs express the necessary receptors for HIV-1 infection including CD4 and CCR5 but they lack DC-specific ICAM3 (intercellular adhesion molecule)-grapping molecule DC-SIGN (CD209) [88]. It is not clear if the LCs are productively infected or are just used by HIV-1 to be transferred to CD4⁺ T cells. Another cDC subtype present in the stroma below the genital epithelium, stromal DCs, on the other hand express all the necessary HIV-1 receptors including DC-SIGN, a C-type lectin receptor binding mannose rich glycoprotein present in pathogen associated patterns (PAMP) on bacteria, fungi and viruses used to sample pathogens and signal to T cells [89]. It has been shown that DC-SIGN binds gp120 and allows uptake of HIV-1 and can lead to the transfer to susceptible CD4⁺ T cells without productive infection of the DC [90].

Memory CD4⁺ T cells expressing high levels of CCR5 are present in the entire epithelium and can be infected directly by HIV-1 or receive virus via contact with LCs or stromal DCs [91].

The role of tissue macrophages in the establishment of the early infection is not clear. CCR5 expressing HIV-1 infected macrophages have been reported in cervical explants studies [92]. Furthermore macrophages are able to uptake HIV-1 particles via macropinocytosis and store the particles for several days before transferring it to susceptible CD4⁺ T cells [93, 94]. Although the extent to which DCs are productively infected in the mucosa and contribute to the initial viral replication is not clear, it has been know that the presence of DCs greatly enhance the replication of HIV-1 in CD4⁺ T cells [95]. This enhancement is due to two mechanisms. DCs are able to activate T cells via MHC class II – T cell receptor (TCR)/CD4 cross talk. T cell activation requires activated (mature) DCs, which have detected an ongoing infection (discussed below). Once the T cells are activated, the presence of NFAT and NF-κB will drive 1 expression as discussed above. The second way DCs enhance HIV-1 replication in CD4⁺ T cells is by increasing de novo infection. As mentioned above this can happen in trans via captured virus either on the cell surface or in cytoplasmic vesicles or by transferring newly produced viral particle after infection. The DCs and CD4⁺ T cells created a very close contact called the virological synapse where virus can easily be transferred to the other cell [96, 97].

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In the first 7 to 14 days the virus replicates and spreads from the initial site of infection to draining lymph nodes. This phase is called eclipse and no viremia or immune response is detected at this point [98]. Viral RNA is only detectable in peripheral blood mononuclear cells (PBMC) after 7 to 21 days and can be detected by PCR. This starts the phase of acute infection (reviewed here [79]).

1.5.2 Acute infection, latency and progression to AIDS

After the initial infection is established viremia reaches the peak with up to 10⁷ copies of viral RNA per milliliter of blood during the first 6 weeks (Figure 1.4). This phase is usually accompanied by flu-like symptoms such as fever, enlarged lymph

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nodes and joint pain. At the time of peak viremia the immune response starts to take effect and HIV-1 specific CD8⁺ T cells and neutralizing antibodies appear. This and the exhaustion of activated CD4⁺ T cells as available host cells lead to a decline in viremia of 100-fold to 1 to 10⁵ copies per ml of blood and the virus enters the latency phase.

During this time the virus replicates on a lower level and the T cell count is steadily declining. Patients can be infected from 1 to 20 years without showing any clinically symptoms until the CD4⁺ T cell count drops from initially 1000 cells per micro liter blood to fewer than 500 to 200 cells and the immune system cannot function properly any longer. At that point opportunistic infections increase leading eventually to the death of the infected patient. If left untreated HIV-1 has a mortality rate of greater than 95%. The viral load is correlated to the speed of progression to AIDS, meaning that the more virus is in the system the faster T cells are depleted and the immune system stops functioning [99]. The virus replication is supported by mostly activated CD4⁺ T cells in the blood and the mucosa. 40% of the lymphocytes reside in the gut associated lymphoid tissue (GALT), which makes up the largest part of the immune system. HIV-1 replication leads to disruption of the intestinal mucosa and destruction of the intestinal epithelial barrier and the leakage of gut bacteria into the patient’s system causing additional inflammation (reviewed here [100]). The virus levels and the rate of infected cells in the blood remain in a stable balance during the latent phase. This means that exhausted cells are getting depleted at the same rate as newly produced cells are getting freshly infected and are supporting the virus population. HIV-1 has an average replication cycle of 1 to 2 days generating more than 300 generations of genomes per year in an infected individual, providing the basis for viral evolution (quasi species). This goes on unless the patient receives the highly active antiretroviral therapy (HAART), consisting of a combination of HIV-1 inhibitors. After the first two weeks of HAART, the viremia drops 10 to 100-fold and virus levels drop below detection limit after 8 to 10 weeks of HAART (reviewed in [101]). More sensitive methods revealed that there is still virus replication ongoing in patients with full therapy and that HAART alone is not enough to remove the virus completely from the system because of the existence of a small fraction of virus producing cells of unknown origin. HIV-1 also takes advantage of the immune system to stay latent in quiescent CD4⁺ T cells. Activated infected CD4⁺ T cells will go

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through rapid expansion and will give rise to effector CD4⁺ T cells. Most of them die quickly but a small fraction will differentiate into memory CD4⁺ T cells and enter a resting state, where the provirus remains in the genome but no viral gene expression takes place until the T cell is activated again [102].

The role of latently infected macrophages is still debated but they might play an important role as a reservoir, especially since infected monocytes are able to cross the blood brain barrier and differentiate into microglia cells, where they reside as tissue specific macrophages ([103-105] and reviewed here [106]). Recently it has been shown that HIV-1 can infect CD34⁺ hematopoietic stem cells (HSC) and they might serve as a long term reservoirs [107]. So far all attempts to re-activate HIV-1 from its reservoirs by CD4⁺ T cell activation with CD3 antibodies or IL2 administration combined with anti retroviral drugs has not proven to be useful to eradicate HIV-1 from infected patients [108].