In health and in EBV lymphoproliferative diseases, GC derived memory B cells and plasma cells constitute the primary functional EBV reservoir [
2,
3,
9]. Hence, by inhibiting GC derived memory B cell and subsequent plasma cell differentiation, EBV loads may be reduced. We have described the immunological effects associated with an elevated CD4
+ IL21
+ profile in an elderly lady suffering from chronic EBV infection where some of the clinical and immunological (severe CD8
+ lymphopenia, B lymphopenia, hypogammaglobulinemia) findings resembled those of B cell chronic active EBV disease (B cell CAEBV) [
10]. However, due to only moderately elevated EBV copy numbers, which is not a typical characteristic of B cell CAEBV [
10], we decided to classify her condition, based on the hematopathologist’s diagnosis, as having high grade EBV reactivation. Based on our calculations, we cannot exclude that non-B cells contributed to the patient’s EBV burden. However, according to our calculations, her B cells contained on average at least twice as many EBV-copies than non-B-cells, consistent with the pathological findings. T- and NK- cell tropic EBV infections are rare entities primarily reported among East Asians (our patient was of Danish ancestry). Our histological examination provided no evidence for clonal T cell expansion. T and NK-cell EBV disease also primarily affects children or young adults who develop symptoms as fever, persistent hepatitis, skin symptoms, uveitis, hepatosplenomegaly or pancytopenia [
11], none of which characterized our patient. Animal studies have shown, that CD4
+ T cell produced IL-21 is crucial for anti-viral CD8
+ T cells [
6] and CD8
+ T cells are critical for controlling EBV [
12]. Hence, the elevated CD4
+ IL-21
+ T cell frequencies, reported here for the first time in the context of human EBV disease, could be seen as an appropriate response to her severe EBV infection.
We found no mutations in the patient’s IL-21, IL-21R, IL-21 promoter or in genes related to EBV disease or CVID. We also excluded auto-immune hepatitis and hepatitis B and C as causes for the elevated patient CD4+ IL-21+ T cell frequencies. Increased IL-21 levels, likely produced by pT
FH, have been observed in type I autoimmune hepatitis [
13] and chronic hepatitis B [
14], however our patient’s
CD4+ IL-21+ T cells were primaly non-pT
FH (CXCR5-)
. Collectively, our findings suggest that our patient’s elevated IL-21 profile was indeed secondary to her pulmonary EBV reactivation. Our patient’s IL-21 producing CD4
+ T cells were dominated by an effector profile (CCR7
−) and showed phenotypically no signs of exhaustion (similar PD-1 expression as controls, data not shown). Collectively, this phenotypic profile was consistent with the CD4
+ IL-21
+ T cells being able to home to and sustain antiviral CD8
+ T cell function in peripheral tissues. Our patient’s CD8
+ lymphopenia is a characteristic also commonly found (44%) among patients with B cell CAEBV [
10]. We excluded that her CD8
+ lymphopenia was secondary to HIV, bare lymphocyte syndrome type I (lack of HLA-class I molecules) [
15], vitamin D deficiency [
16]
, CD8A variants or other genetic variants associated with primary immunodeficiency. Furthermore, she had a late onset infectious history and her middle aged son had normal CD8
+ T cell counts and no EBV reactivation. Collectively, this points to a secondary CD8
+ T-cell penia, however one fully consistent with the effects of IL-21, since IL-21 causes accumulation of virus specific effector CD8
+ T-cells in peripheral tissues [
17]. Actually, IL-21 induces expression of the gut homing receptor integrin α
4β
7 consistent with the intestinal accumulation of CD8
+ T cells [
17]. Accumulation of CD8
+ T cells in lymph nodes and spleen is also observed in IL-21 transgenic mice [
18]. Our patient’s modest EBV copy number elevations also indicated some preserved CD4
+ and CD8
+ T-cell functionality in her peripheral tissue. Despite the increased intra- and extracellular IL-21 levels (which likely explained her monocytosis), the patient generated neither pT
FH nor protective protein specific antibodies, notwithstanding the central importance of IL-21 for both (p) T
FH differentiation [
19] and GC derived antibody formation [
5]. T follicular helper cells (T
FH) are located in the GC and are critical for the formation of GC derived memory B cells [
20] and plasma cells [
21]. As a proxy marker for GC located T
FH, we focused on their peripheral counterparts: (CD4
+ CD45RA
− CXCR5
+ CCR7
lo PD-1
hi) pT
FH since there are functional and developmental connections between T
FH and pT
FH, implying that peripheral expansion of pT
FH, one week post-vaccination, correlates with GC derived antibody formation [
8]. Furthermore, both the differentiation of T
FH and pT
FH is dependent on the GC transcriptional repressor Bcl-6 [
8]. Hence, our patient’s lack of pT
FH was consistent with her reduced peripheral CD4
+ Bcl-6
+ T cell frequencies.
Priming and differentiation of CD4
+ IL-21
+ T cells takes place in lymph nodes, consistent with the increased CD4
+ IL-21
+ T cell frequencies, observed seven days post-vaccination (Fig.
3). IL-21 is released within the GC, or in peripheral tissues, and is bound by local IL-21R
+ cells (B cells, CD4
+ and CD8
+ T cells). This might explain why not all patient sera contained elevated IL-21 levels. While having the most pronounced CD4
+ IL-21
+ induction among all vaccinees, the patient had absolutely no Bcl-6 dependent pT
FH induction day 7. Substantiating a potential antagonism between Bcl-6 dependent pT
FH differentiation and the differentiation of CD4
+IL-21
+ T cells, we observed a strong inverse correlation between pT
FH and CD4
+ IL-21
+ formation, pre- and post-vaccination, in patient and controls. This could indicate that other IL-21 producing CD4
+ T cells subsets [
22], apart from pT
FH /T
FH [
8], might either directly compromise the development of the latter or represent alternative GC-derived differentiation pathways. In agreement with this inverse correlation, patient CD4
+ IL-21
+ T cells were predominantly CXCR5
− and hence of a non-pT
FH phenotype. We have not been able to address whether constantly elevated IL-21 levels per se compromised pT
FH /T
FH formation. Although IL-21 is a Bcl-6 stimulator [
5], IL-21 can also suppress intra-nodal Bcl-6 through signal transducer and activator of transcription 3 (STAT3) induced Blimp-1 expression [
23]. It can therefore not be excluded, that the patient’s constantly elevated CD4
+ IL-21
+ levels could have suppressed intra-nodal, Bcl-6 dependent, pT
FH and T
FH induction [
24] and thereby compromised the patient’s memory B cell formation [
20] and subsequent plasmablast generation [
21].
Interleukin-21 also induces apoptosis in resting and activated B-cells [
7] consistent with the widespread B cell lymphopenia and hypogammaglobulinemia observed in this patient and in a large fraction (42%) of B cell CAEBV patients not treated with rituximab [
10]
. Collectively, our data suggests several mechanisms whereby a prolonged increased IL-21 profile might reduce EBV loads: 1) apoptosis of resting and activated B cells combined with compromised GC derived 2) memory B cell and 3) plasma cell differentiation, secondary to disruption of Bcl-6 dependent pTFH formation. Due to the very low frequencies of circulating patient memory B cells, the estimation of EBV content in this subset as well as in plasma cells (which are bone marrow resident) is technically not feasible. However, as the B cells constituted the dominant reservoir in our patient, a reduction in memory B cells and plasma cell formation could be a possible mechanism to reduce EBV loads.
In addition, our patient’s circulating CD4
+ CD25
high FoxP3 T
regs frequency was markedly reduced compared to adult controls - a finding potentially attributable to the T
reg inhibitory effect of IL-21 [
25]. As T
regs can inhibit CD8
+ effector T cell function [
26], reduced T
regs frequencies could aid CD8
+ T effector cell efficacy against EBV but could also increase the risk of collateral tissue (lung) damage due to unconstrained CD8
+ effector T cell activity. The latter might explain her deteriorating lung function.
Consistent with the aforementioned anti-viral mechanisms, the patient had only moderately elevated EBV copy numbers. We can only speculate, as to why our patient developed high grade EBV reactivation, but her homozygosity for HLA-A* 01, a well-known risk allele for EBV+ tumors [
27] could be implicated as well as clonal EBV escape due to extended HLA-class I homozygosity.