There is increasing evidence that microglia serve as antigen presenters in the human CNS. Although the occurrence of MHC class II immunoreactive cells has been reported in astrocytic gliomas, the relative contribution of microglia to this cell population has not been studied in detail. Using computer-assisted image analysis, we have investigated the expression of MHC class II molecules and of the microglia/macrophage markers Ki-MIP, RCA-1, KP1 and iba1, in 97 astrocytic gliomas comprising all WHO grades to answer the question whether there is a correlation between tumour grade and the number of MHC class II positive microglia/macrophage profiles. Microglia expressing MHC class II were common in astrocytomas and anaplastic astrocytomas but rare in pilocytic tumours although there was significant variation within each group. MHC class II immunoreactivity was reduced in highly cellular areas of glioblastomas where large numbers of cells expressing macrophage markers were still present. Thus, there was no simple relationship between tumour grade and microglial/macrophage MHC class II expression. In addition, up to 55% of astrocytic gliomas contained MHC class II immunoreactive tumour cells. Microglia but not tumour cells were found to express the BB1/B7 costimulator. We conclude that microglia in astrocytic gliomas are well equipped to function as antigen presenting cells. Yet, neoplastic astroglia appear to acquire the capacity to downregulate microglial MHC class II expression and, at the same time, may induce T-cell clonal anergy through aberrant expression of MHC class II molecules.

3 4 Running title 5 MYD88 splice variants in B cell lymphoma 6 7 Key points 8 ▪ In human B cells the TLR adaptor and oncogene MYD88 preferentially gives rise to NF-κB-9 promoting canonical splice variants. 10 ▪ B cells lack NF-κB-restriction by alternative MYD88 splicing, a critical negative feedback 11 loop in myeloid cells, favoring lymphomagenesis. 12 13 Abstract 14 Gain-of-function mutations of the TLR adaptor and oncoprotein MyD88 drive B cell 15 lymphomagenesis via sustained NF-κB activation. In myeloid cells, sustained TLR activation 16and NF-κB activation lead to the induction of inhibitory MYD88 splice variants that restrain 17 prolonged NF-κB activation. We therefore sought to investigate whether such a negative 18 feedback loop exists in B cells. Analyzing MYD88 splice variants in normal B cells and 19 different primary B cell malignancies, we observed that MYD88 splice variants in 20 transformed B cells are dominated by the canonical, strongly NF-κB-activating isoform of 21 MYD88 and contain at least three novel, so far uncharacterized signaling-competent splice 22 isoforms. TLR stimulation in B cells unexpectedly reinforces splicing of NF-κB-promoting, 23 canonical isoforms rather than the 'MyD88s', a negative regulatory isoform that is typically 24 induced by TLRs in myeloid cells. This suggests that an essential negative feedback loop 25 restricting TLR signaling in myeloid cells at the level of alternative splicing, is missing in B 26 cells, rendering B cells vulnerable to sustained NF-κB activation and eventual 27 lymphomagenesis. Our results uncover MYD88 alternative splicing as an unappreciated 28 promoter of B cell lymphomagenesis and provide a rationale why oncogenic MYD88 29 mutations are exclusively found in B cells.

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