Persistent Human Papillomavirus Infection

Abstract

Persistent infection with oncogenic human papillomavirus (HPV) types is responsible for ~5% of human cancers. The HPV infectious cycle can sustain long-term infection in stratified epithelia because viral DNA is maintained as low copy number extrachromosomal plasmids in the dividing basal cells of a lesion, while progeny viral genomes are amplified to large numbers in differentiated superficial cells. The viral E1 and E2 proteins initiate viral DNA replication and maintain and partition viral genomes, in concert with the cellular replication machinery. Additionally, the E5, E6, and E7 proteins are required to evade host immune responses and to produce a cellular environment that supports viral DNA replication. An unfortunate consequence of the manipulation of cellular proliferation and differentiation is that cells become at high risk for carcinogenesis.

Keywords: HPV; cancer; epithelium; extrachromosomal replication; immune evasion; latency; papillomavirus; persistence; tethering.

Figure 1

Natural history of oncogenic human papillomavirus infection. A model showing the progression of HPV infection to invasive cancer. Infection with HPVs is usually cleared by the immune system within a couple of years. Persistently infected cells can regress, but over time can progress to invasive cancer.

Figure 2

Map of the HPV genome. Schematic representation of an Alphapapillomavirus genome. The green, purple, and blue arrows represent the early, accessory, and late viral open reading frames, respectively. The upstream regulatory region (URR) shown in grey contains regulatory elements including the origin of replication (ori) that contains binding sites for the E1 and E2 replication proteins (denoted by a green square and circles, respectively). The early (P_E), late (P_L) and E8 (P_E8) promoters, and the early (pA_E) and late (pA_L) polyadenylation sites are indicated.

Figure 3

HPV infectious life cycle. Schematic representation of the differentiated layers of a stratified epithelium infected with HPV. The virus accesses the basal keratinocytes through a microabrasion. Upon cellular entry, the virus is trafficked through the endosome and enters the nucleus (encased in a membrane vesicle) following breakdown of the nuclear membrane during mitosis. Within the nucleus, HPV genomes localize to promyelocytic leukemia nuclear bodies (PML-NBs), undergo a limited round of DNA synthesis and become established by tethering to host chromatin to maintain the viral genome at a constant copy number in dividing cells. Upon epithelial differentiation, infected cells amplify the viral DNA to high copy numbers, and late viral genes are expressed for virion assembly/packaging. Virions are sloughed from the epithelial surface in viral-laden squames. The different steps in the viral lifecycle are summarized below the schematic and the corresponding viral protein expression levels indicated on the right.

Figure 4

Phases of HPV replication. A plot showing the different phases of HPV replication in the host epithelium. Upon entry, HPV genomes undergo a limited burst of DNA amplification. The viral genome becomes established in the nucleus, and in the maintenance phase is replicated at a low copy number and partitioned to daughter cells. In differentiated cells, the viral DNA undergoes a second burst of amplification to a very high copy number to generate genomes for progeny virions. A model of viral partitioning during the maintenance phase is illustrated; HPV genomes (green circles) attach to host chromosomes (blue) and are partitioned to daughter cells during mitosis. The mitotic spindle is shown in dark blue.

Figure 5

Cellular processes modulated by the high-risk HPV accessory proteins. Schematic representation of the multiple strategies employed by the E5, E6, and E7 accessory proteins to establish a cellular environment in tissue-specific niches that supports viral replication and persistence and evade immune surveillance. Abbreviations: cGAS–STING, cyclic GMP–AMP synthase–stimulator of interferon genes; EGFR, epidermal growth factor receptor; IFN, interferon; NF-κB, nuclear factor κB, PDZ, postsynaptic density protein, disc large tumor suppressor, zonula occludens-1 domain-containing proteins; pRb, retinoblastoma protein. Images created with BioRender.com.

Figure 6

Replication proteins and elements. (A) The structural and functional domains of the E1, E2, and E8^E2 proteins. (B) The upstream regulatory region (URR) of the Alphapapillomavirus HPV18, illustrating the cis-elements required for stable viral genome maintenance. The minichromosome maintenance enhancer element (MMEE) and origin of replication (ori) are indicated. The E2 binding sites are represented by green circles and the E1 binding site by a green square. The late polyadenylation site pAL and early viral promoter P_E are also shown.