**3.2. Other diagnostic markers**

Comprehensive phenotypic characterization of PTLD reveals potential reliance on EBV or NF-kappaB signaling instead of B-cell receptor signaling. Several signaling pathways, cells of origin of PTLD, and their relation to viruses were analyzed by immunohistochemistry and in situ hybridization. Most PTLDs are of activated B-cell origin. Two-thirds of cases show an Epstein-Barr virus (EBV) infection of the neoplastic cells. NF-kappaB signaling components are present in the majority of cases, except for EBV-infected cases with latency type III lacking CD19 and upstream B-cell signaling constituents. Proteins involved in B-cell receptor signaling like Bruton tyrosine kinase are seen only present in a minority of cases. Phosphoinositide 3-kinase (PI3K) is found to be expressed in 94% of cases and the druggable PI3K class 1 catalytic subunit p110 in 76%, while other signal transduction proteins are expressed only in occasional cases. Unsupervised cluster analysis has revealed three distinct subgroups: (I) related to EBV infection, mainly latency type III and lacking CD19, upstream B-cell signaling, and NF-kappa constituents; (ii) related to EBV infection with expression of the alternative NF-kappaB pathway compound including RelB, CD10, and FOXP1 or MUM1; and (iii) unrelated to virus infection with expression of the classic NF-kappaB pathway compound p65 [12]. EBV and NF-kappaB are important drivers in PTLD in contrast to B-cell receptor signaling. The main signal transduction pathway is related to PI3K. This links PTLD to other subgroups of EBV-related lymphomas, highlighting also new potential treatment approaches [4].

The diagnosis of PTLD relies on comprehensive morphologic examination, immunophenotyping, genetics, and EBV status. Most of PTLDs are of B-cell origin. EBV plays an important role in the pathogenesis of PTLD. The duration of disease onset is shorter in EBV-positive cases.
