Tumour infiltrating lymphocyte density differs by meningioma type and is associated with prognosis in atypical meningioma.

Tumour infiltrating lymphocyte (TIL) density is prognostically significant in various tumours, but few studies have investigated its significance in meningioma. This study aimed to investigate how TIL density differs by meningioma histology and whether it is a predictor of meningioma recurrence. We studied CD3, CD8, CD4, FOXP3 and PD-1 positive (+) TIL density in a continuous cohort of 476 meningiomas resected at Auckland Hospital between 2002 and 2011 using tissue microarrays and computer assisted image analysis. TILs were identified in all meningiomas except one (median CD3+ TIL density across entire cohort 53.0 cells/mm). Most TILs were CD8+ (median 33.6 cells/mm) with smaller numbers of CD4+ TILs (median 2.9 cells/mm). PD-1+ (median 0.32 cells/mm) and FOXP3+ (median 0.0 cells/mm) TILs were scarce. Reduced CD3+ (p=0.0066), CD8+ (p=0.0029) and PD-1+ (p=0.0375) TIL density was seen in WHO grade II/III meningioma compared with WHO grade I. Pairwise comparison confirmed statistically significant differences in TIL density existed between meningioma types (CD3, CD8, CD4, p<0.0001; FOXP3, p=0.0096; PD-1, p=0.0090) with chordoid meningioma having the lowest overall CD3+ TIL density (median 12.5 cells/mm). Despite its low TIL density, chordoid meningioma had a higher FOXP3:CD8 ratio than several meningioma types. Atypical meningioma had a higher FOXP3:CD8 ratio than transitional meningioma (p=0.0045). No association between TIL density and recurrence was seen across the entire cohort or by WHO grade. However, CD3+ and CD8+ TIL density was associated with recurrence in atypical meningioma on multivariable analysis (CD3, p=0.0012; CD8, p=0.0071). A higher CD3+ and CD8+ TIL density was associated with improved recurrence free survival. Our findings suggest CD3+ and CD8+ TIL density is prognostically significant in atypical meningioma. Further investigation of this observation and its biological basis is warranted. The differences in TIL density by meningioma histology may be of relevance in studies of therapeutic immune checkpoint inhibition.