HIV infects CD4+ T cells and cells of macrophage lineage. It causes lytic and subsequently latent infection of CD4 T cells (as well as syncytia formation) and persistent low-level productive infection of macrophages. 

How HIV Crosses the Epithelium

Mucosal transmission of HIV The multilayered epithelium may act as a physical barrier to repel most of HIV, and it is only those few viral particles that are able to penetrate into the lamina propria that can initiate an infection. How does the virus make it through the barrier? The stratified epithelium of the vagin is not the tight, impenetrable barrier of the skin. Although algandular, it is moist, and fluid is continuosly passing through the intercellular space form the lamina propria. Epithelial cells are connected by discotinous patches of desmosomes, the weakest form of intercellular junction. It is possible that a small fraction of the viruses in the inoculum may be able to permeate through the entire epithelium to the lamina propria. Another possibility is that the virus may bring Langerhans cells in the peithelium without infecting them. Mechanisms by which HIV may traverse the epithelium may also include tears in the epithelium, transcytosis. It is possible that for transmission to occur, transport of virus across the epithelium to the lamina propria is a limiting factor in infection efficiency. Viruses such as poliovirus, herpesvirus, and rhinovirus have been shown to directly infect epithelial cells, which may explain why these viruses are more easily transmitted than HIV.

In the submucosal space HIV may infect local T cells or macrophages that express CD4 and either . Virus entry can be blocked by agents that prevent binding of the viral Env protein to either CD4 or the viral coreceptor. However, HIV may still be able to establish an infection by binding to CD4, DC-SIGN, or other attachment molecules on the surface of DCs. HIV may be internalized by the DCs, and upon CD maturation (which may be triggered by HIV) and emigration from the submucose via the lymphatics to regional lymph nodes, be delivered to an area rich in T cells but deficient in topically applied entry inhibitors.

Molecular Mechanism of  HIV entry

The first step in HIV infection is binding of viral gp120 to receptors on target cells. The major cellular receptor for HIV is CD4. Because the T4 cell expresses the highest levels of CD4,  HIV is often called “lymphotrophic.” But there are other cells that bind HIV such as macrophages, monocytes,, langerhans cells, hematopoic stem cells, certain rectal lining cells, and microglial cells. These cells express low levels of CD4 and thus can bind less HIV than T4 cells. 

Although the primary receptor for HIV is CD4, viral entry also requires the presence of a co-receptor, either  (a receptor for B-chemokines) in the case of macrophage tropic HIV strains, or(a receptor for alpha-chemokines) in the case of of T tropic HIV strains (Macrophages are suseptible only to HIV variants that use CCR5 for entry whereas T cells are most efficiently infected by variants that use CXCR4). During the primary and asymptomatic stages of infection, the predominant HIV-1 strain is one that uses the CCR5 chemokine coreceptor, which as stated is found primarily on macrophages (M-tropic HIV-1). The primary ligand for the HIV co-receptor CCR5 is , also called . Copy numbers of this ligand has been reported to vary among diverse human populations and linked to susceptibility to HIV infection. HIV-positive groups were over-represented by individuals having fewer copies of CCL31L. Statistical analyses also linked higher gene copy numbers with a lessened risk of HIV progression. 

As the infection proceeds, a switch in the viral phenotype gives rise to a CD4+ T-lympohocyte-tropic isolate that uses the  (T-tropic HIV-1) resulting in a steep decline in CD4+ T cells and an AIDS diagnosis. CCR5 is the receptor for primary HIV and SIV previously referred to as nonsyncytium-inducing or macrophage tropic viruses. Binding of gp120 to CD4 alone does not trigger membrane fusion, but causes conformational changes in gp120, which allow binding to the . 

HIV variants transmitted by sexual contact are macrophage-tropic and use  as a coreceptor. The viruses that are found within the first few months after infection usually require Individuals with a homozygous polymorphism in the CCR5 gene consisting of a 32 base pair deletion (delta32 CCR5) in the coding region do not express CCR5 on cell surfaces and are generally resistant to infection by M tropic HIV-1.

How did one come to discover these co-receptors? Well, in the case of   agonists (normal ligands for CCR5 like  ) were able to block infection of macrophage targets. Similarly, in the case of CXCR4 agonists (like SDF-1) were found to block infection of T cells. Both the CCR5 and CXCR5 coreceptors belong to the family of G protein-coupled 7-transmembrane-segment receptors, which all contain an N-terminal region that is acidic and tyrosin rich. 

There appears to be an additional membrane receptor called CD26 which has protease activity and binds to another site on gp120. The binding of CD26 to gp120 is thought to release an HIV fusion protein from gp120 which allows the previously buried hydrophobic fusion peptide to insert into the plasma membrane of the cell. The fusion protein then spontaneously rearranges and the energy released is used to pull the membrane which encloses the viral genome with the plasma membrane of the cell together.

There is a relationship between the virus load in an HIV-infected person and transmission to an uninfected partner.

After HIV binds to its receptor, the envelope fuses with the cell membrane and the HIV nucleocapsid is internalized. The cell takes up the virus in the cytoplasm in a vesicle. In the cell, the RNA genome is released through processes not completely understood. 

Infection of Myloid Dcs

Initial transmission of HIV-1 R5 strains can occur by infection of mucosal epithelial cells through the galactosylceramide and CCR5 receptors and transmigration of virus to reach submucosal MDCs and CD4pos T cells. This could also occur by selective transcytosis of R5 virus through the mucosal cells. A third possible route is extension of dendrites by submucosal MDCs between epithelial cells and the exterior, thus rendering the MDCs susceptible to direct infection by HIV-1.

More recently, a new pathway was revealed for HIV-1 interaction with DCs. Rather than attaching and fusing to DCs via its glycoprotein spikes and primary and secondary cell receptors, HIV-1 enters DCs by the bindign of gp120 oligosaccharides to mannose C-type lectin receptors (CLRs), such as langerin (CD207), mannose receptor (CD206) and DC-SIGN. A protion of virus bound to CLRs is rapidly taken into endolysosomal vacoules, where it is maintained for days in an infectious state. Another significant portion of virus, hwoever, appears to be rapidly degraded in non-endolysomsomal compartments.

Contact of these DCs with CD4pos T cell localizes a high concentration of internalized virus to an infectious synapse on the DC surface with the CD4 primary receptor and chemokine coreceptor on the T cells. This leads to transfer of virus to the T cells and highly productive virus replication. Notablfy, expression of Nef, a non-enzymatic immunomodulating protein of HIV-1 affects this pathway by upregulating DC-SIGN epxression and promoting clustering of CD4pos T cells around the infected DCs.

Drugs which May Enhance HIV entry

Marijuana ( may enhance HIV- infection of susceptible cells. Also, HIV is able to active the and HIV-1 infection of human lymphocytes and moncytes/macrophages is greatly enhanced by opsonization of the virus with complement. 

Cocaine has reportedly increased DC specific C type ICAM-3 grabbing nonintegrin (DC-SIGN) expression of DCs. Caoaine using long term nonprogressors and noraml progressors of HIV infection manifested significantly higher levels of DC-SIGN compared with cocaine nonusing long term nonprogressors and normal progressors.

From RNA to DNA: 

Once the HIV RNA has been introduced into a target cell and uncoated, the RNA is reversed transcribed into DNA using reverse transcriptase. Transposones (also known as jumping genes) can encode reverse transcriptases.The nucleotides used are generated in the cytoplasm. Reverse transcriptase starts to transcribe the RNA of the HIV virus in the cytoplasm of the host cell. In order to begin synthesis the reverse transcriptase requires a starting point provided by a specific TRNA found in the host and also in the virus. 

The fidelity of the reverse transcriptase is rather low and about 3-10 mistakes can occur per genome after any reverse transcriptase reaction. This high mutation rate leads to a greater degree of variability in the encoded viral proteins which helps the virus escape recognition and possible neutralization by the immune system. In fact, HIV is capable of tremendous genetic variation, with mutations in the viral genome occurring at rates millions of times faster than what is observed in human DNA. Sequencing studies reveal that no two AIDS patients carry the identical virus! Moreover, isolates taken form the same individual at different times also can differ substantially. These changes make development of an HIV very difficult because antibodies or cell mediated immunity directed against one isolate may not recognize another isolate. When HIV is grown in T cell lines in the presence of human serum containing neutralizing antibody, a viral population resistent to the neutralizing antibody emerges after 4-5 weeks in culture.

Obtain a hybrid RNA-DNA molecule after reverse transcription (one strand RNA and newly synthesized DNA strand). This molecule is not fit for integration into the host molecule. The transcriptase has the ability to degrade the remaining RNA strand in the RNA-DNA hybrid and then replace that strand with another DNA strand. As a result a double stranded DNA molecule is formed that contains all the viral information. 

Integration into Host DNA: 

After only 10 hours after entry into the cell, the viral genome is integrated into the host genome. Specific DNA sequences near the 2 ends of the double stranded DNA product produced by reverse transcriptase are held together by a virus encoded integrase enzyme. This integrase creates activated 3 OH viral DNA ends that can directed attack a target DNA molecule through a mechanism very similar to that used by cut and past DNA  Integration occurs in random position in host genome. Once integrated, the viral DNA is permanently associated with the host cell DNA and is passed on to daughter cells as the cell divides. 

Transcription: 

latent reproduction: In some cases, there may be weeks to years of latency. With every replication of the host cell, the virus is duplicated and passed on to daughter cell. Macrophages serve as the reservoir for the virus because they may survive for many weeks with the virus. However, HIV may propagate right after integration of its genome into host. 

Transcription of HIV proviral DNA into mRNA is catalyzed by RNA  which initially binds to the promoter in the 5′ LTR. The HIV promoter is relatively weak and has a low affinity for RNA polymerase. Synthesis of full length transcripts can occur only after this weak promoter is converted into an active one. Thus activation of the provirus from latency into the lytic state depends on this conversion. Several factors have been shown to influence this conversion. 1) binding of the HIV regulatory protein Tat to the Tar element in the emerging RNA transcript mediates this promoter conversion. 2) certain host cell transcription factors such as NFkB bind the HIV promoter. Certain viruses like cytomegalovirus, herpes simplex virus, Epstein Barr virus, papovavirurses and Hepatitis B virus can induce production of such transcription factors. These findings explain why IV drug users who are often coinfected with other viruses progress to AIDS very quickly.

HIV mRNAs are processed like usual cellular mRNAs which means they are cut or ligated. This process if known as splicing and offers the possibility of storing many different genes within a rather small viral genome. This is achieved by combining different portions of the primary transcript and thereby generating from a single transcript various mRNAs which code for proteins with different features and activities.

Protein Synthesis: After processing, the mRNAs are transported into the cytoplasm. The cytoplasmic ribosomes first synthesize viral proteins which control propogation (proteins which selectively promote synthesis of viral RNA). Later capsid proteins are also synthesized which are needed for packaging of viral genomes. Finally, the anchor proteins are synthesized at ribosomes which are attached to the ER. 

ER is the cellular compartment where synthesis of proteins targetted for the plasma membrane takes place. Together with membrane components of the ER, the mature viral surface proteins are transported to the PM where viral assembly is initiated. 

Proteins which will be located in the viral lipid layer are positioned in the PM where also future capsid proteins accumulate. Finally, the copies of the viral genome are added.

The cytomplasmic membrane then invaginates or  buds out. It can be observed by electron microscopy. This budding can in itself spread the virus when it binds to CD4 molecules on the surface of another cell. This can lead to a fushion cell. 

In the end, the bud and future virus is released from the cell. This happens simultaneously at locations on the PM. A single infected cell may produce thousands of viral particles. Only mature virus is capable of infecting new host cells.

Immediately after budding, an HIV protease (inside the virus) must cut the virus in 2 positions yielding 3 fragments which then form the actual capsid. Only the mature virus is capable of infecting new host cells.