Expression of Tim-1 and Its Pathogenetic Role in Primary CNS Lymphoma

Primary central nervous system lymphoma (PCNSL) is a subtype of non-Hodgkin’s lymphoma that arises within the central nervous system (CNS) as a primary lesion, most of which demonstrate diffuse large B-cell lymphoma (DLBCL) histology. However, CNS is recognized as an “immune sanctuary”, and it is not clear in what mechanism B cells develop tumor at this immunoprivileged site. In the past mouse models of multiple sclerosis and cerebral infarction, regulatory B cells, a population of B cells with high IL-10 producing capacity, were reported to have a function to migrate to CNS and suppress inflammation. As the IL-10 level is typically increased in the cerebrospinal fluid (CSF) of PCNSL patients, we hypothesized that PCNSL might originate from B cells that have a physiological role to produce IL-10 for suppressing unfavorable inflammation in CNS, such as regulatory B cells in mice.

Recently, a cell surface molecule T cell immunoglobulin domain and mucin domain protein 1 (Tim-1) has been reported to be specifically expressed in the majority of regulatory B cells in mice. Tim-1 was originally identified as a costimulatory molecule on T cells that negatively regulates cellular immune response. Regulatory B cells in mice with defective Tim-1 mutation were reported to demonstrate a profound defect in IL-10 production, suggesting that Tim-1 plays an essential role in their IL-10 production. However, there has been no previous report on Tim-1 expression on human B-cells or B-cell lymphomas, or what function they may serve if it is present.

We performed immunohistochemical staining of Tim-1 in various formalin-fixed paraffin embedded lymphoma samples. We observed strong expression of Tim-1 in PCNSL samples, in contrast to its lower expression in other DLBCL and follicular lymphoma samples. Expression of Tim-1 is also detected in a cell line derived from PCNSL (TK), as well as in several other B-cell lymphoma cell lines, by RT-PCR and western blot.

As we detected spontaneous shedding of the ectodomain of Tim-1 in the culture media of Tim-1-expressing B-cell lymphoma cell lines, we examined whether Tim-1 can also be detected in the CSF of PCNSL patients. By ELISA, we detected soluble Tim-1 in the CSF of PCNSL patients with active disease, and found it undetectable after the successful treatment with chemo/radiotherapy. The level of soluble Tim-1 in the CSF was positively correlated with IL-10, and it is suggested that these two molecules are functionally related also in humans. In a patient with continuous Tim-1 detection in the CSF after chemotherapy while radiological examination could no longer detected any abnormality, subsequently manifested relapse in the brain. According to these findings, soluble Tim-1 in CSF may be expected to serve as a sensitive biomarker for PCNSL.

To clarify the biological role of soluble Tim-1 in tumor microenvironment, we examined its effect on T cell function. We stimulated CD4⁺ and CD8⁺ T cells with or without soluble Tim-1 in vitro and compared their cytokine production. The result showed that, in the presence of soluble Tim-1, both IL-2 and IFN-g production was suppressed in CD8⁺ T cells. In conclusion, Tim-1 is expressed in PCNSL and shedding of extracellular domain of Tim-1, in addition to IL-10, may contribute to the immunosuppressive microenvironment of PCNSL.