1. The Primary Target: CD4+ T Cells
* HIV primarily targets CD4+ T cells, which are crucial for the immune system's function. These cells express a protein called CD4 on their surface, which serves as the primary attachment site for HIV.
2. The Co-Receptor: CCR5 or CXCR4
* In addition to CD4, HIV needs a second protein called a co-receptor to gain entry into the cell. The most common co-receptor is CCR5, which is expressed on macrophages and some T cells.
* Some strains of HIV, particularly those associated with later stages of infection, can also use CXCR4, found on T cells.
3. Attachment and Fusion
* HIV's gp120 protein, located on the viral envelope, binds to the CD4 molecule on the T cell. This binding is highly specific and initiates the attachment process.
* After attaching to CD4, gp120 changes its shape and interacts with the co-receptor (CCR5 or CXCR4). This interaction brings the viral envelope and the cell membrane closer together.
* The viral envelope protein gp41 then undergoes a conformational change, extending a fusion peptide that inserts into the cell membrane. This triggers fusion of the viral envelope with the cell membrane, allowing the viral contents to enter the cell.
4. Entry and Replication
* Once inside the cell, HIV releases its RNA genome and enzymes. These enzymes reverse transcribe the viral RNA into DNA, which integrates into the host cell's genome.
* The integrated viral DNA then directs the host cell to produce new viral components. These components assemble into new HIV particles, which bud off from the infected cell and go on to infect other cells.
Key Points:
* The process of HIV attachment to host cells is highly specific, relying on the presence of both CD4 and a co-receptor.
* This specific attachment process is crucial for HIV's ability to infect and spread within the body.
* Understanding this attachment mechanism is essential for developing anti-HIV therapies that target the entry process and prevent viral infection.
