Discussion
The results of our study add significantly to the limited data available
for pediatric patients. This is due to the high percentage of relapsed
patients in our cohort and to the high number of markers analyzed8-10,15. We found an association between PD-L1
expression and interim remission status, which is not reported in the
literature before. PD-L1 plays an important role in the pathogenesis of
cHL. It is upregulated through amplification of chromosome 9p24.1 and
may support cHL proliferation 18. PD-L1 also
contributes to the immune escape of the tumor 19. This
may explain why patients with cHL respond so well to PD-1/PD-L1
checkpoint inhibition 19-21. Our findings imply that
PD-L1 expression in pediatric cHL is correlated with outcome. This is an
important finding that should be further investigated in a larger cohort
of pediatric patients.
Remarkably, there are some differences between our data and previously
published findings in children with cHL (See Supplemental Table S5 for a
summary of the findings of previous studies). Barros et al. found a
significant association between the expression of CD163 and the
progression-free survival (PFS) 8, however, our study
and the study of Gupta et al. found no significant association. Gupta et
al. and Dinand et al. found a significant association between CD30 and
CD15 expression and EFS 9,15. We were not able to
confirm this finding. These differences may be caused by differences in
treatment protocols between the different studies. Second, our study
contained a much higher percentage of events than the study of Barros
and Dinand 8,15. Third, expression patterns in our
study were based on lager areas and exact percentages were calculated.
Interestingly, our results are in contrast with previous findings in
adult patients. They mostly reported an adverse association between OS
and EFS and the expression of PAX5 on the HRS cells, and CD68+, CD163+,
PD-1+, and PD-L1+ in the TME 4-7. These differences
between pediatric cHL and adult cHL may be due to differences in
pathogenesis, resulting from differences in the composition of the
TME10-14. Moreover, differences may be due to the use
of different antibodies and/or scoring methods.
We found high expression of CD30 and PD-L1 on HRS cells, and PD-1 and
PD-L1 in the TME in all patients. CD30, PD-L1 and PD-1 are all markers
that can be targeted therapeutically (e.g. by brentuximab vedotin
binding to CD30). The expression in pediatric patients underscores the
need for further investigations of these novel therapies in first line
treatment in children.
Our study is the first to examine eight different markers
simultaneously. Another strength of our data is the event-enriched
setting particularly in pediatric patients. However, due to this
event-enriched setup, we included patients diagnosed over a period of 18
years with different treatment regimens. Despite this event-enriched
cohort, the study still lacked the statistical power to perform a
sub-analysis per treatment protocol for the primary outcome.
A possible limitation of the study is the use of immunohistochemical
staining. This can lead to inconsistent results depending on differences
in tissue fixation, the type of antibody and staining method, the
scoring method, and the observer’s interpretation. To minimize these
inconsistencies, we based our scoring method on published previous
studies 4,7,14,22,23 and we tried to overcome the
interobserver variability by using a dual-head microscope and reviewing
tissue samples together.
In conclusion, we found an association between PD-L1 expression and
interim remission status, which should be further investigated.
Furthermore, our data demonstrate differences in the expression patterns
and prognostic impact of immunohistochemical markers between pediatric
and adult patients with cHL. Further research into these differences may
lead to specific prognostic factors in pediatric cHL, indispensable for
improvement of treatment in this population.