A review of the risk factors associated with juvenile-onset recurrent respiratory papillomatosis: genetic, immune and clinical aspects

Shi-Lan Li 1, · Wei Wang · Jing Zhao 1, · Feng-Zhen Zhang 1, · Jie Zhang 1 · Xin Ni

Abstract

Keywords Epidemics · Genetic · Human papillomavirus infection · Immune · Recurrent respiratory papillomatosis · Risk factors

lntroduction

Recurrent respiratory papillomatosis (RRP) is a viral disease predominantly caused by chronic infection with lowrisk human papillomaviruses (HPVs) 6 and 11. The age distribution of RRP is trimodal, with the first peak in children younger than 5 years of age, the second peak in adults between 20 and 40 years, and the third peak in individuals approximately aged 64 years [ 1, 2]. When the disease occurs in children, it is referred to as juvenile-onset recurrent respiratory papillomatosis (JoRRP). JoRRP is the most common benign lesion of proliferating respiratory epithelial tissue in children [ 3].

The disease course of JoRRP is variable. The number of surgeries, interval between surgeries, and extent of disease involvement of the airways vary widely between patients.Unlike recurrent respiratory papilloma in adults (AoRRP),JoRRP does not become malignant and is self-limiting with age. However, recently several articles have reported a few patients develop malignancy even in adulthood [ 4– 6]. The reason for malignant transformation may be related to the age at diagnosis, HPV type, and multiple resections [ 4].Therefore, understanding the factors affecting the disease course is important for disease prevention and assessment of disease severity, patient prognosis, and treatment modalities. In this paper, we will review the studies of JoRRP,and describe the risk factors associated with the immune,genetic, and clinical aspects in the context of recent studies.

Epidemiologicacl haracteristics of juvenile-onset recurrent respiratory papillomatosis

General

Children with JoRRP are usually first diagnosed at the age of 2–3 years [ 4, 7]. Males and females are equally affected.The incidence rate varies greatly among different countries.It is estimated at 4.3/100,000 [ 8], or 80–1500/year [ 9, 10]in the United States of America (USA), and it is similar in Canada [ 11], 1.38/100,000 in South Africa [ 12], and only 0.17/100,000 in Norway [ 13]. Pathologically, JoRRP mainly manifests as papillary hyperplasia of respiratory squamous epithelium. Owing to its highly recurrent nature, children with JoRRP require frequent surgical treatment. Some patients have even undergone more than one hundred operations in their lifetime [ 14], thus imposing a serious economic and healthcare burden on the physical and mental health of the child and their families. In the USA, patients with JoRRP undergo an average of 4.4 surgeries annually, and over 40 surgeries in a lifetime, amounting to $60,000–$470,000 [ 15].Furthermore, frequent recurrences of JoRRP increase the likelihood of spread of the lesion to the lower respiratory tract, and can lead to carcinogenesis of the larynx [ 4].

Transmission

HPV infection is highly specific and only infects the human skin and mucous membranes, leading to tissue proliferation that induces warty lesions and papillomas. Unlike HPV-16 and HPV-18 subtypes that cause cervical cancer, JoRRP is related to the low-risk subtypes, HPV-6 and HPV-11 [ 16,17].

Three modes of transmission are suggested in children:vertical transmission at birth (although HPV type concordance between the mother and the newborn in different studies are contradictory [ 18– 20]), vertical transmission in utero[ 21], and horizontal transmission via the child’s environment[ 18].

The most convincing research is a case–control study of 3033 Danish infants. The JoRRP risk in children whose mothers had genital warts during pregnancy increased by 231.4 times, compared to those whose mothers had no warts.Furthermore, the risk of acquiring JoRRP doubled when the mother had a spontaneous vaginal delivery lasting more than 10 hours [ 22]. This study also suggested that mothers are likely to transmit HPV to their infants during spontaneous delivery, resulting in JoRRP. However, it is still controversial whether cesarean delivery protects against the occurrence of JoRRP. Elective cesarean sections reduce vertical transmission of herpes simplex, human immunodeficiency virus, and hepatitis B, but the role of elective cesarean section in preventing HPV transmission remains controversial [ 23]. Some studies suggest that HPV transmission may still occur in utero or postpartum even if cesarean delivery is performed[ 15, 24]. Vertical transmission can occur at any point from the sperm at fertilization [ 25] to delivery and may explain part of the phenomenon. In the first weeks of life, when there is a close physical relationship requiring close contact between the infant and the mothers, relatives, caregivers, and fomites, HPVs can be horizontally transmitted to the child.

Recently, additional evidence has indicated that HPV is very stable, survives outside the host for several days, and many currently applied environmental disinfectants are ineffective against the HPV [ 26]. A prospective study in Spain found that infants born to HPV-positive mothers and those born to HPV-negative mothers tested HPV-positive at some point during the infants' follow-up [ 18]. Contaminant transmission, inoculative transmission, and nosocomial infections may contribute to infecting children with HPV.

為了確保學(xué)生能夠及時有效地完成好課程內(nèi)容，智慧樹網(wǎng)絡(luò)聯(lián)盟課程中一共有7次章測試及1次期末考試安排，需督促學(xué)生按時完成，其總成績占學(xué)生最后成績的70%。除此之外，在教學(xué)過程中面授課的考勤占10%，課堂討論評分占20%，這3個部分構(gòu)成了學(xué)生的最終成績。建立這種過程性考核與結(jié)果性考核機制是一個較好的考核方式[4-5]，能讓學(xué)生在相互交流、觀摩中指出存在的問題，加深對理論知識的理解，領(lǐng)悟不同知識點之間的聯(lián)系。

In addition, HPV infection alone is not enough for the occurrence of JoRRP. Although HPV was detectable in the nasopharyngeal secretions of 30% of infants with a history of obstetric virus exposure [ 27], only a few developed the disease. Other factors, such as a patient's immune condition;genetic susceptibility; duration, length, and amount of viral exposure; or open wound/mucosal exposure may be important determinants of JoRRP occurrence [ 24].

Genetic factors associated with the course of juvenile-onset recurrent respiratory papillomatosis

Human leukocyte antigen (HLA)

The HLA

complex is the most polymorphic genetic complex in humans. Most of these genes encode proteins involved in regulating the function of the immune system, and there is also a clear relationship between gene polymorphisms and the genetic susceptibility of a patient to the development of disease [ 28]. It has been reported that HLA class II genes help protect the body from pathogens such as viruses and are associated with human susceptibility to infectious diseases[ 29]. Therefore, understanding

HLA

gene polymorphisms in JoRRP patients not only complements current knowledge regarding immune mechanism but also helps provide new ideas for the study, diagnosis, and treatment of the disease.At present, different countries have researched this aspect and found that HLA class II genes are increased in RRP patients, including

HLA

-

DRB1

*

0102

,

HLA

-

DRB1

*

0301

,and

HLA

-

DQB1

*

0201

[ 30]. The presence of

DRB1

*

0301

was associated with more severe disease, as pure congeners of this gene were found in patients with severe disease[ 31]. The size of the virus-like particle (HPV-11 L1 VLPs)response observed in JoRRP patients was found to be negatively correlated with the patients' Derkay scores [ 31]. The

DRB1

*

11:01

and

DRB1

*

11:01

-

DQB1

*

03:01

haplotypes are strongly associated with disease susceptibility to severe RRP in Koreans [ 32]. A study of Chinese Han children found that the frequencies of

HLA

-

DRB1

*

03:01

and

HLADQB1

*

02:01

alleles were higher in children with JoRRP than in controls [ 33].

Human papillomavirus 6/11 E6 E7

The

HPV

genome encodes six early (E) proteins that are mainly associated with viral gene regulation and cellular transformation, the most important of which are E6 and E7,which are consistently expressed in all malignancies caused by HPV and collaborate to cause epithelial cell immortalization [ 34]. Their expression and the number of transcripts directly determine the likelihood of malignant transformation of cells [ 35]. Detection of

HPV E6

/

E7

mRNA expression in cervical cells can accurately assess the extent and prognosis of cervical intraepithelial lesions [ 36]. The

HPV 6

/

11 E6

/

E7

RNA probe was applied for the first time in JoRRP patients for colorimetric in situ hybridization, and tumor tissues were found to be positive in all patients [ 14].This confirmed the presence of

HPV

transcripts in the tumors. Furthermore, patients with higher levels of E6 and E7 transcripts were found to have more aggressive disease.This may indicate that the more active viral transcripts are in local tissues, the more frequent is the disease recurrence.

Study on T-cell immune changes related to juvenile-onset recurrent respiratory papillomatosis course

The widespread presence of HPV with a low incidence of JoRRP suggests the opportunistic nature of the HPV viral pathogenesis. Some studies have found a high incidence of HPV-related diseases in individuals with iatrogenic (such as xenografts) or acquired cell-mediated immunodeficiency (such as acquired immune deficiency syndrome) [ 37– 39]. For children, the critical period for the development of the immune system occurs between 4 and 6 years of age [ 40]; the period exactly covers the first age distribution peak of RRP. Therefore, we speculate that the immature young immune system can lead to decreased antigen processing and/or presentation or decreased secretion of pro-inflammatory cytokines, which may be related to the severity of JoRRP. In addition, the course of JoRRP is variable; some patients experience spontaneous remission, whereas others may continue to spread down the airway [ 10], suggesting that the occurrence and development of JoRRP may be closely related to individual immunity.

T-lymphocytes are an important group of immune cells that play an essential role in the antiviral process. T cells,derived from lymphoid progenitor cells in the bone marrow, undergo differentiation, development, and maturation in the thymus and migrate to peripheral lymphoid tissues.Depending on the expression of the cluster of differentiation (CD), mature T cells are classified as CD4+ T cells and CD8+ T cells. According to their different functions in the immune response, they are divided into multiple functional subsets, such as helper T cells (Th), cytotoxic T cells, and regulatory T cells (Tr or Tregs) [ 41]. There is evidence that altered proportions or dysfunction of subsets of T cells may contribute to disease progression in JoRRP.

CD4+ and CD8+ T cells

When the number and function of T lymphocyte subsets are abnormal, immune disorders and diseases may arise,which may be one of the reasons for the recurrence of JoRRP [ 42]. However, according to current research,the number of CD4+ and CD8+ T cells in patients with JoRRP remains controversial. Several previous studies have shown that the ratio of CD4+ and CD8+ T cells in the peripheral blood of JoRRP patients did not differ from that of healthy individuals, and the CD4+/CD8+ ratio in tumor tissues was not significantly different [ 43– 45]. This suggests that specific immune dysregulation rather than overall immunodeficiency occurs in patients with JoRRP.Recently, researchers have found that the percentage of cytotoxic T cells relative to Th cells in the peripheral blood of patients with JoRRP is significantly increased,resulting in the inversion of the CD4+/CD8+ ratio (< 1),and that the cytokine gene spectrum of patients with the lowest ratio is generally inhibited in monocytes, Th cells,cytotoxic T cells, and natural killer cells [ 46]. The reason for this finding is unclear; the small sample size of the study flow cytometry at different times to address phenotypic changes over time should be studied to evaluate the reason for the inverted ratios.

Helper T cells

Th1 and Th2 cells

Th cells are a subset differentiated from initial CD4+ T cells and have the function of assisting T and B lymphocyte response. Under the action of different factors, Th cell precursors can shift and differentiate into Th1 or Th2 cells. Although the changes in CD4+ and CD8+ T cells in the peripheral blood of patients with JoRRP are still under investigation, the ability of phorbol-12-myristate-13-acetate to stimulate CD8+ T cells to secrete interferon-gamma is not impaired, resulting in increased interleukin (IL)-4 levels in the peripheral blood, suggesting that there may be an enhanced Th2-like response [ 45, 47]. In addition, the number of CD8+ CD28? T cells in a patient's peripheral blood is significantly increased compared to that in healthy people; thus, it is possible that these might block HPV-specific cytotoxic Th1 T-cell development through the production of immunosuppressive factors, such as IL-4, IL-10, and transforming growth factor beta (TGF-β) [ 44, 48]. Therefore,JoRRP has a biased adaptive immune response, manifested by an enhanced Th2-like response and a diminished Th1-like response [ 43].Further studies on the chemokines of Th2-like response revealed that the ratio of Th2/Th1 chemokines in patients with RRP was increased and correlated with the severity of the disease. A study found that the inducible Th2-like chemokine C–C motif ligand 20 (CCL20) was selectively expressed in the basal keratinocyte layer of a papilloma,which reaffirmed the presence of a Th2-like predominance in RRP. In addition, the plasma levels of Th2 chemokines CCL17, CCL18, and CCL22 in patients decrease with the clinical remission of the disease [ 49], suggesting that Th2 chemokines in the patient's plasma may become a powerful tool as a prognosticator for this disease.

Regulatory T cells

Tregs are a population of specialized CD4+ T cells with immunosuppressive functions that are essential for the maintenance of immune tolerance and immune homeostasis in the body [ 53]. There are at least two major types of Tregs in humans: one is the naturally occurring Tregs produced in the thymus with a CD4+ CD25+ CD127 low-Fox P3+ phenotype, which have the ability to suppress responses of both CD4+ CD25? and CD8+ CD25? T-cells in a contactdependent, cytokine-independent, and antigen non-specific manner; the other type includes the adaptive or induced dendritic cells (iTregs), which are produced in the surrounding tissues and enriched in the tissues. iTregs consist of at least three subtypes, including type I Tregs (CD4+ CD25-IL10+ FoxP3low/?), Th3 cells that are dependent on IL-4 for functional differentiation, and peripherally induced Tregs that express FoxP3. All of these secrete the same cytokine IL-10 and/or TGF-β to exert their inhibitory activities [ 54].Tregs are upregulated in many different cancer tissues. A study on RRP found that Tregs (CD4+ CD25+ CD127low-FoxP3+) in papillary tumor tissues are 2–7 fold higher than the Tregs in a patient's autologous blood, but there was no significant change in the level in the peripheral blood [ 55].Thus, Treg levels are also higher in RRP tissues. In addition, the percentage of CD8+ CD28?T cells in the tissues of patients with RRP was positively correlated with disease severity, with the percentage increasing proportionally with disease progression [ 43]. In recent years, CD8+ CD28?T cells have been extensively investigated. Owing to the lack of CD28+ expression, they have immunosuppressive activity, such as CD4+ CD25+ Tregs, and are also an important group of Tregs, which can inhibit the proliferation of T cells and inhibit their cytotoxicity. We speculate that the infiltration of Tregs in papillomas may inhibit the Th1-like response against HPV in papilloma tissue and enable chronic HPV infection and persistent recurrence of papillomas.

In addition, impaired Treg function is also considered a possible mechanism for the development of RRP [ 56].Experiments have shown that Treg-like immunosuppression could occur after the initial T cells were induced to express FoxP3 in vitro or in vivo, indicating that FoxP3 is a key factor for controlling the expression of immunosuppressive molecules. In turn, polymorphisms in the promoter region of the

FoxP3

gene can affect the function and number of FoxP3 molecules, leading to defective Treg function [ 57, 58]. In a Korean study of 30 RRP patients (26 AoRRP and 4 JoRRP)who underwent gene sequencing of the peripheral blood, it was found that the genotype frequencies of rs5902434 ATT/ATT and rs2232365 GG of

FoxP3

genes were significantly decreased in female patients compared to healthy controls,suggesting that

FoxP3

gene polymorphisms could be an important protective factor in the immune tolerance to HPV infection in the pathogenesis of RRP [ 59].

Other factors infulencing JoRRP

Human papillomavirus genotype

The HPV genotype can affect the interval and time of recurrence of RRP. Researchers tested patients' tumor tissues and revealed more aggressive disease in those infected with HPV-11 than in those infected with HPV-6 [ 11, 14,60– 64]. To provide an overall assessment of disease severity, all these studies followed the Doyle et al. [ 62] criteria as previously reported. The criteria for aggressive JoRRP were defined as the total number of operations ≥ 10, average number of operations per year ≥ 4, distant metastasis, or tracheotomy. These criteria have been cited and used in many other studies [ 63– 66]. Chinese scholars also validated this issue and obtained similar conclusions as those from other international studies comparing double HPV 6/11 subtype infection to a single low-risk HPV-11 subtype infection,wherein the latter presents with a younger age of first onset and more metastatic disease [ 67]. In addition, the detection rate and types of HPV genotypes can change in the same individuals at different times. The reason remains unclear but may be related to the different locations of laryngeal samples. A recent cohort study found that the HPV genotype is more applicable for evaluating the course of JoRRP than AoRRP. Studies have shown that patients with HPV-11 subtypes in JoRRP had more aggressive disease than those with HPV-6 subtypes, but there was no difference in AoRRP [ 68].

Age at first onset

In recent years, several studies have concluded that the younger the age at first presentation, the greater the total number of surgeries and the annual number of surgeries [ 10,69, 70]. An initial onset age of less than 5 years is a relevant factor for the extralaryngeal spread of JoRRP [ 71]. Based on the multicenter study of Buchinsky et al. [ 7] age is a more important factor associated with the aggressiveness of disease than the HPV genotype. The results showed that 80% of the disease course was aggressive in patients with an age of onset of 0–5 years compared to 30% in patients with age of onset of over 10 years. Moreover, accounting for confounding between HPV-11 and young age, HPV type was minimally associated with aggressiveness of the disease.A recent non-concurrent cohort study also observed that JoRRP required more surgical procedures and had higher

Derkay scores than AoRRP [ 68]. Therefore, age is a more important factor affecting the prognosis of RRP.

Socioeconomic status

Socioeconomic status (SES) has been identified as a risk factor for cervical cancer, which is also a disease secondary to HPV infection [ 71]. Populations with a lower SES may be more likely to engage in high-risk sexual activities or delay access to medical screening services, resulting in higher exposure and infection rates. The same conclusion was also found for JoRRP. The results of a study conducted on 257 patients with JoRRP in China demonstrated that a patients'family income and parents' educational level were related to the incidence of disease [ 72], with lower family income and education levels being associated with higher disease incidence. This suggests that the incidence of JoRRP is related to SES. Whether SES affects disease severity remains controversial. In a 10-year prospective study in the USA,approximately half of the 73 JoRRP patients were covered by Medicaid, representing a low SES indicator compared to private insurance coverage. Using a multivariate linear regression analysis that was conducted using peak severity score and peak surgical frequency as the outcomes of interest, the study revealed that Medicaid coverage was significantly correlated with more aggressive disease [ 16]. However, in another study of 603 cases from 22 tertiary pediatric care centers in the USA grouped by Medicaid coverage, no significant difference was found in the frequency of surgery[ 73]. In addition, a study of 21 patients from a single center in Canada and a joint USA-Canadian multicenter study of 118 patients using total family income and maternal education as indicators of SES demonstrated that SES was not significantly associated with JoRRP severity [ 42, 69]. One possible explanation is that although a large proportion of patients were socioeconomically vulnerable, the availability of universal access to the health care system of these researched regions can provide support. Thus, it is unclear whether high SES plays a role in facilitating prompt access to medical care and improving prognosis. The differences in findings may be related to the different regions, different health insurance policies, and insufficient sample sizes. It appears that an improved health care system is important to prevent JoRRP.

All the risk factors mentioned above and their relationship with JoRRP are shown in Table 1.

Conclusions

JoRRP is characterized by multiple recurrences and aggressive disease. Currently, effective treatment for JoRRP is lacking. Mother-to-child transmission is the most important mode of disease transmission. Genetic, immunological and several clinical factors play an important role in the course of JoRRP. In Genetic aspect, a number of studies about HLA class II genes and HPV 6/11 E6/E7 have been conduct and found to have a correlation with the severity of JoRRP. The immunology of JoRRP is the focus of the current research and has been illuminated much. Patients of JoRRP develop specific immune disorders instead of global immunodeficiency. Some T-cell immune factors have been found to be associated with disease severity. However, the mechanisms of the disease remain unclear, and some of the results are controversial.In the future, we need to explore a more accurate biomarker to predict the prognosis. Furthermore, a more complete understanding of the immunological basis is needed to explain why some HPV-infected individuals develop chronic disease, while others do not, which would be helpful to develop therapeutic vaccines to treat JoRRP.

Table 1 Gene, immune and clinical factors associated with juvenile-onset recurrent respiratory papillomatosis

human leukocyte antigen, human papillomaviruses, helper T cells, cluster of differentiation, interleukin, C–C motifligand, regulatory T cells

Classify Factors of research In JoRRP Relationship with disease severity(positive correlation factor or negative correlation factor)Genetic factors The White race: HLA-DRB1*0102, HLA-DQB1*0201, HLA-DRB1*0301 Increased HLA-DRB1*0301 (positive)Korean: DRB1*11:01 and DRB1*11:01-DQB1*03:01 haplotypes Increased Positive Chinese Han children: HLA-DRB1*03:01 and HLA-DQB1*02:01 Increased No research Expression of the RNA of HPV 6/11 genes E6 and E7 Increased Positive Immune factors In the peripheral blood: Th2 cell, CD8+ CD28- T cell, IL-4, CCL17, Increased CCL17, CCL18, CCL20 (positive)CCL18, CCL20 Th1 cell, IL-21 Decreased IL-21 (negative)Clinical factors In tissue: Tregs, CD8+ CD28? T cell, CCL20 Increased CD8+ CD28? T cell (positive)HPV 11 Positive Younger age at first onset Positive Socioeconomic status Lower Controversial

Acknowledgements

I would like to thank the editors and reviewers for their dedication to the manuscript. The comments on the manuscript are helpful for both the manuscript and future research.

Author contributions

ZJ and NX participated in the conceptualization of the study and edited the manuscript. LSL wrote the original draft.WW had modified the part about immune and genetic. ZJ and ZFZ had participated in building the framework, and modified the part about clinical. All authors read and approved the final manuscript.

Funding

This work was supported by the Special Fund for the National Natural Science Foundation of China (81970867) and Beijing Hospitals Authority’ Ascent Plan (DFL20191201).

Data availability

All data are available and the correspondent can be contacted if requested.

Declarations

Ethical approval

Not needed.

Conflict of interest

Author NX is a member of the Editorial Board for

World Journal of Pediatrics

. The paper was handled by the other Editor and has undergone rigorous peer review process. Author NX was not involved in the journal's review of, or decisions related to, this manuscript. No financial or non-financial benefits have been received or will be received from any party related directly or indirectly to the subject of this article. The authors have no conflict of interest to declare.