Expression of different L1 isoforms of Mastomys natalensis papillomavirus as mechanism to circumvent adaptive immunity

Abstract

Although many high-risk mucosal and cutaneous human papillomaviruses (HPVs) theoretically have the potential to synthesize L1 isoforms differing in length, previous seroepidemiological studies only focused on the short L1 variants, co-assembling with L2 to infectious virions. Using the multimammate mouse Mastomys coucha as preclinical model, this is the first study demonstrating seroconversion against different L1 isoforms during the natural course of papillomavirus infection. Intriguingly, positivity with the cutaneous MnPV was accompanied by a strong seroresponse against a longer L1 isoform, but to our surprise, the raised antibodies were non-neutralizing. Only after a delay of around 4 months, protecting antibodies against the short L1 appeared, enabling the virus to successfully establish an infection. This argues for a novel humoral immune escape mechanism that may also have important implications on the interpretation of epidemiological data in terms of seropositivity and protection of PV infections in general.

Isoforms of the Papillomavirus Major Capsid Protein Differ in Their Ability to Block Viral Spread and Tumor Formation

“…Green M: methionine upstream of the Wx7YLPP motif initiates L1 SHORT .Red M: upstream in-frame methionine of L1 LONG . Figure modified from ( 14 ).…”

“…For that purpose, 8-week-old virus-free Mastomys coucha (six animals per group) were vaccinated four times in biweekly intervals until week 6, followed by experimental MnPV infection at week 10 (outlined in Figure 2 ) and followed for up to 52 weeks (continued in Figure 4 ). Since our previous study showed that kinetics of seroconversion and specificity of antibodies against L1 LONG and L1 MIDDLE variants were similar ( 14 ), we only compared the impact of L1 LONG and L1 SHORT vaccination on subsequent viral challenge and skin tumor formation.…”

“…In these animals, the temporal and spatial spread of the cutaneous MnPV ( Mastomys natalensis papillomavirus) can be investigated, starting from early infection until the final manifestation of a skin tumor in an immunocompetent host ( 13 ). Like humans, the animals become infected early after birth, which is accompanied by seroconversion against several viral antigens ( 14 , 15 ). Notably, MnPV-infected Mastomys mimic many aspects of human skin carcinogenesis like the aforementioned cooperation of infection and UV exposure in SCC development via a hit-and-run mechanism ( 16 ).…”

“…Independently from the tissue tropism (either mucosal or cutaneous), a remarkable feature of many PV types causing serious clinical manifestations is the occurrence of alternative in-frame initiation codons (ATGs) upstream of the canonical L1 open reading frame (ORF; from here on referred to as L1 SHORT ), potentially allowing the translation of different isoforms of the major capsid protein ( 14 ). While only this L1 SHORT is capable to form genuine infectious virus particles and VLPs ( 14 , 18 ), the biological function of other L1 isoforms and immune responses against them was hitherto unknown. Therefore, we recently measured time course and specificity of the humoral immune response in naturally MnPV-infected animals.…”

“…Therefore, we recently measured time course and specificity of the humoral immune response in naturally MnPV-infected animals. Remarkably, seroconversion against the long L1 variant (L1 LONG ) preceded that against L1 SHORT by about four months, but only antibodies against the latter were neutralizing in in vitro assays and therefore protective ( 14 ). This time gap apparently enables successful MnPV infection and viral spread in the natural host, arguing for a humoral immune escape mechanism.…”