The Human Papillomavirus (HPV) is a small, non-enveloped DNA virus that commonly infects epithelial cells. Its life cycle is unique because it is completely dependent on the differentiation status of the host skin or mucosal cell it infects. HPV utilizes the natural processes of cell maturation and migration to replicate its genetic material and produce new infectious particles. The virus executes a sequential, multi-stage process, beginning with entry into the basal layer and concluding with the passive release of new virions from the surface.
Gaining Entry and Nuclear Transport
The infection begins when the HPV virion encounters micro-abrasions in the epithelial surface, gaining access to the underlying basal cells. The virus first attaches to the host cell surface by binding its major capsid protein (L1) to primary receptors, such as heparan sulfate proteoglycans (HSPGs). This attachment triggers a conformational change in the virion, exposing binding sites for a secondary receptor. The virion is then internalized, often through an endocytic pathway that can be clathrin- or caveolin-mediated.
Once inside the cell, the viral genome begins its journey toward the nucleus. The L1 capsid protein is shed (uncoated), and the viral DNA remains complexed with the minor capsid protein L2. This L2-DNA complex traffics through the cytoplasm, utilizing the cell’s internal transport system, including the microtubule network.
The nuclear envelope is a barrier for the L2-DNA complex, and entry into the nucleus is often restricted until the host cell undergoes mitosis. During mitosis, the nuclear membrane naturally breaks down, allowing the viral genome access to the nuclear domain for transcription and replication. Upon nuclear entry, the viral DNA typically localizes near promyelocytic leukemia (PML) nuclear bodies, where gene expression is initiated.
Early Phase: Genome Maintenance and Transcriptional Control
After the viral genome is transported into the nucleus, the early phase begins with the expression of the early genes (E1, E2, E6, E7, etc.). The primary goal is the stable maintenance of the viral genome as a low-copy-number extrachromosomal element, known as an episome. The genome is initially amplified to a copy number ranging from 10 to 200 per cell to establish the infection.
Two early proteins, E1 and E2, are central to this maintenance phase. E2 acts as a sequence-specific transcription factor and binds to the viral origin of replication. It then recruits the E1 protein, which functions as the viral helicase, unwinding the DNA to engage the host cell’s replication machinery. E1 and E2 together form the pre-initiation complex necessary to replicate the viral DNA.
In addition to initiating replication, E2 regulates transcription and partitions the viral genome during cell division. E2 tethers the viral episome to the host cell’s mitotic chromosomes, ensuring that each daughter cell receives a copy of the viral DNA when the basal cell divides. This regulated replication and segregation allows the viral genome to persist in the basal layer.
Late Phase: Amplification and Structural Protein Synthesis
The shift from the maintenance phase to productive replication is linked to the differentiation of the infected cell as it moves upward through the epithelial layers. This change in cellular environment triggers high-level amplification of the viral genome, known as vegetative replication. Hundreds or thousands of viral genome copies are synthesized in the nucleus of these differentiating cells, far exceeding the low copy numbers maintained in the basal layer.
This replication is facilitated by the continued expression of E1 and E2, and by the action of the E6 and E7 early proteins. E6 and E7 manipulate the host cell cycle, co-opting the host’s S-phase machinery. E7 targets the tumor suppressor protein pRb (retinoblastoma protein), leading to its degradation and pushing the cell into the S-phase. E6 complements this action by targeting the tumor suppressor p53 for degradation, preventing cell death or cycle arrest in response to the E7-induced proliferation.
The expression of the late genes, L1 and L2, marks the late phase. These genes encode the structural proteins of the viral capsid and are produced in the upper, terminally differentiated layers of the epithelium. L1 is the major component and L2 is the minor component; both are required for the final construction of the infectious virion.
Assembly and Viral Egress
Assembly involves the L1 and L2 structural proteins accumulating in the nucleus. L1, the major capsid protein, self-assembles into a nearly complete capsid shell. The viral DNA is then packaged into these newly formed capsids within the nucleus.
The packaging process involves a size discrimination mechanism that preferentially incorporates the 8,000 base pair viral DNA. The L2 minor capsid protein guides the viral genome into the pre-formed L1 shell.
Unlike many viruses that cause rapid cell lysis, HPV virions are released in a slow, passive process known as egress. The virus does not actively burst the cell. Instead, the mature virions are released as the terminally differentiated host cells (squamous cells) naturally shed from the surface of the epithelium, a process called desquamation.