1. Introduction
Epstein Barr Virus (EBV) has been recognized for its ability to transform B lymphocytes and its association with different types of cancers such as Burkitt lymphoma (BL), Hodgkin lymphoma HL, nasal NK-T cell lymphoma, and B-cell NHL.[
1,
2] The time course of EBV acquisition differs in sub-Saharan Africa from that in developed countries. Seroprevalence is nearly universal throughout sub-Saharan Africa with acquisition in early childhood, in developed countries with acquisition later in life coincident with sexual activity.[
3,
4,
5] Although the majority of the world’s population is infected with EBV, there are global variations in the incidence of EBV-associated cancers, perhaps resulting from host genetics, environmental factors, or viral genetic variation, with EBV having a striking and geographically unique association with BL and HL in sub-Saharan Africa.[
6] In western countries, about 50% of all cases of cHL are EBV positive. In comparison, in developing countries the percentage in pediatric cHL is approaching 90-100%.[
7,
8,
9,
10,
11,
12] In sub-Saharan Africa, EBV has been detected at high rate among HL cases.[
13] Similarly, a study comparing the epidemiological variation of EBV in HL for Kenyan and Japanese patients showed that EBV-encoded RNA was [
14] found in 79% and 59% of the Kenyan and the Japanese cases, respectively.[
15]
In recent years, the microenvironment has become a major focus of attention, having a potential role in the pathogenesis of HL with prognostic and therapeutic implications.[
16,
17] The HL microenvironment in the tumor tissue is rich with reactive inflammatory cells, including T-cells.[
18] However, the immune cells present in the tumor do not impose an effective anti-tumor immune response, but may instead support the growth and proliferation of HRS cells.[
19] The abundance and role of T-helper cells has continued to be a controversial topic in the microenvironment of HL. Some studies have suggested a Th-1 mediated response to be predominant.[
18,
20] Greaves
et al. showed that the predominant T-lymphocyte in the microenvironment of HRS cells is of Th-1 subtype being activation marker-rich, cytokine-secretory and proliferative rather than being Th-2 and T-reg.[
21] A recent study indicated that the majority of cytotoxic CD8+ T cells in EBV+ cHL are present due to a Th-1 response as Th-1 cells stimulate a cellular immune response.[
22] Other studies have suggested a Th-2 polarization. [
23,
24] Immunosuppressive regulatory T cells (Treg) have also been identified in CHL tissue.[
20] It has been suggested that a local accumulation of Th2 cells and Treg,[
20,
23] provide an explanation as to how HRS cells may escape anti-tumour immunity
Programmed cell death 1 (PD1), the immune checkpoint,[
25] was found to be expressed on tumor infiltrating lymphocytes.[
26] One of the immune escape mechanisms of CHL is the expression of PD-L1 on the HRS tumor cells.[
27,
28] However, studies on the PD1 expression in the microenvironment of cHL are scarce and conflicting.[
27,
29,
30] Overall, the cellular infiltrate in cHL appears to play a decisive role in allowing the HRS cells to survive by providing an environment that suppresses cytotoxic immune responses.[
31]
The aim of this study is to evaluate the EBV prevalence among HL cases in Ethiopia. The HL microenvironment has been studied using high throughput techniques in developed countries. Since the prevalence of EBV with HL is different between developed and resource-limited countries, this study aimed to determine the cellular component of the microenvironment of EBV-related and EBV-unrelated HL using IHC in patients from this Sub-Saharan African country.
4. Discussion
The association of EBV with HL has been established. Although EBV seroprevalence was high in Sub-Saharan Africa countries, studies related to EBV association with HL is scarce. The goal of current study was to evaluate expression of EBV markers in HL and determine associations with epidemiological factors and immune biomarkers in the microenvironment of HL. EBV positivity highly age dependent, with the highest fraction in younger HL patients, with a preponderance among males. In this study, cHL cases in the group of children and young adults (age ≤24 years) were most often associated with EBV (80%). This finding is in consistence with results from Kenya and the Middle East [
11,
34] and in opposition to what has been reported from Western countries.[
35,
36] In contrast to the developed countries, EBV infection in developing countries acquired during the early age of life[
37]. We found that male HL cases were more often associated with EBV, confirming previous finding.[
36,
38] The sex variation in EBV infection between males and females may either be due to a higher rate of HL among males, as has been reported previously or due to sex genetic variation.[
39,
40,
41] Also, female hormones are reported to protect against EBV infection.[
9] In the present series, the MCCHL subtype was more often associated with EBV infection, followed by NSCHL, while all NLPHL cases were negative for EBV. The expression of LMP1/EBER has been associated with different clinicopathological features of HL, such as sex, age, and HL subtypes.[
36]
EBV has been considered a risk factor for several types of lymphoma, including HL. However, the prevalence of EBV with HL has shown different association patterns in developing and developed countries. In the current study, EBV was detected in 66.4%, in line with findings from other sub-Saharan countries.[
6] In USA and Europe, 30%-50% of HL cases are associated with EBV,[
9] while 60-100% of HL cases have been associated with EBV in Asian and African countries.[
15] The prevalence and age variations in EBV infection between developed and developing countries are perhaps due to socioeconomic, hygiene, geographical and genetic differences.[
6,
42,
43]
The expression of the biomarkers used in this study, T-cells and T-cell subsets, have been confirmed and reported previously in several studies. For instance, the transcription factor C-maf has been used as an identification marker for Th2 in several series.[
44,
45] The transcription factors, FoxP3, T-bet and C-maf predominantly have been expressed on T-reg, Th1 and Th2 cells in the TME of HL.[
24] In addition T-bet and C-maf have been detected in the HRSc.[
45] In the TME of HL (cHL & NLPHL), the frequency of CD4+ cells was two times higher than the CD8+ cells, also in line with earlier reports.[
46,
47] Daussy and co-workers have described the CD4:CD8 ratio in lymphoma; their study revealed that the CD4 cell population was higher than the CD8 cell population in HL, a result comparable to our findings. In contrast, the CD4:CD8 shows an inverted ratio in diffuse large B-cell lymphoma DLBCL.[
48]
In the TME of both types of HL (the cHL and the NLPHL), the expression of T-reg, Th1 and Th2 biomarkers (the CD4+ cell subsets) showed a significant variation. Most cells in the CD4+ population were of the Th2 subset, followed by the Th1 and T-reg subsets. The abundance of C-maf+ cells in the TME could reflect the polarization of CD4+ cells to be differentiated into the Th2 subset. The TME of cHL is reported to be high in C-maf+ and FoxP3+ cells.[
24] The tumor cells of HL release signals to manage the type and activities of cells in their microenvironment, ensuring their proliferation, existence and escape immune surveillance.[
24] The T-reg transcription factor (FoxP3) expression was higher among the cHL subtypes. In contrast, the number of PD1-expressing cells was higher among NLPHL. Possibly, the quantity and type of CD4+ cell subsets in the TME of HL might differ depending on the presence of EBV infection. Male sex was associated with higher expression of FoxP3 expression, a finding similar to what has been reported previously.[
49]
Our study shows that the type and proportion of immune cells in the TME are highly influenced by the presence of EBV in the tumor cells (LMP1/EBER) of the classical Hodgkin lymphoma (cHL) cases. The abundance of CD4+ cells was similar in the microenvironment of EBV-related and EBV-negative cHL cases, as reported previously.[
50] In contrast, the CD8+ cell population was higher in the EBV-related HL group. Foxp3 and T-bet, the transcription factors of T-reg and Th1 cells, were expressed in high levels in the microenvironment of EBV-related HL. The abundance of CD8 and T-bet, the biomarkers of T-cells related to the cellular-mediated immune response was high in EBV-related cHL. However, it is possible that the concomitant recruitment of cytotoxic-T-cells and Th1 with T-reg cells in the TME of EBV-related HL, would suppress the cytotoxic activities of CD8+ T-cells and Th1 cells, thereby ensuring the persistence and proliferation of the HRSCs. In line with our finding, a recent study indicated that HL with EBV was dominated by CD8+ T-cells and a mileu rich inTh1 cytokines.[
22,
51] EBV infection has a role in altering the immune component of the microenvironment of HL. EBV might be involved in the genetic dysfunction of the tumor cells; LMP1 constitutively activates several signaling cascades and pathways in the tumor cells,[
52,
53] which can induce the production of cytokines and chemokines used for the recruitment of various type of immune cells.[
54]
An interesting finding was that while the EBV-related cHL was rich in Foxp3+ cells, the TME of EBV-unrelated cHL was rich in PD1+ cells. The differential recruitment of the T-cells (Foxp3+ vs PD1+) with immune suppressive/regulatory activities shows a differential strategy of HRSCs to escape the immune surveillances in the presence or absence of EBV. The difference in the cells expressing Foxp3 between the EBV-related and EBV-unrelated TME is significant, a result contradicting what has been reported previously.[
22] Similar to what was reported by Wu and coworkers, an association of T-reg cells with cHL has been detected in the current series of cases. Our finding regarding the recruitment of T-reg in high quantity in the TME of EBV-related HL is consistent with what reported from Brazil[
55] and France[
56] and contradicted with reports from Germany.[
51] High PD1+ cells with EBV-related breast cancer has been reported.[
57] No difference was reported by Duffield and coworkers in the abundance of PD1+ cells between the EBV-related and unrelated HL,[
51] The difference between our studies is in the method used for scoring the immune cells signature and the larger sample size used for this study.
A strength of our study is that of using an image analysis platform to quantify the expression of biomarkers in the TME of HL. Since the HL cases were from a Sub-Saharan country, the high association of EBV with HL made it possible to compare the microenvironment of EBV-related and EBV-unrelated cases. Due to resource limitation, we didn’t conduct multicolor immunohistochemistry.
Author Contributions
Conceptualization, M.A., Y.B., R.H., B.P., M.J. and A.G.; methodology, Y.B. and A.G..; validation, M.A. and Y.B.; formal analysis, M.A. and M.J.; writing—original draft preparation, M.A., R.H., M.J. and B.P.; project administration, B.P.; funding acquisition, M.J. All authors have read and agreed to the published version of the manuscript.