B lymphocytes in the normal brain: contrasts with HIV-associated lymphoid infiltrates and lymphomas

In recent years evidence has accumulated which suggests that the brain may not be the immunologically privileged site it was once considered to be. It is now widely accepted that T lymphocytes perform surveillance functions in normal brain parenchyma. However, as yet there are no reports of B lymphocytes entering brain parenchyma in the healthy state. This study aimed to determine first the prevalence of B lymphocytes in normal brain, and subsequently whether advancing HIV infection led to changes in the brain B lymphocyte population, which might contribute to the increased risk of lymphoma seen in AIDS. Our results show that B lymphocytes do enter all parts of the normal human brain in very low numbers and that the B lymphocytes within the brain parenchyma display an activated (CD23 positive) phenotype. In contrast, intravascular B lymphocytes have a much lower expression of activation markers. B lymphocytes were found in increased numbers in both the brain parenchyma and perivascular spaces of pre-AIDS brains. However, brains from the majority of AIDS subjects, including those with primary CNS lymphoma (PCNSL) (outside the area of neoplastic involvement) contained fewer B lymphocytes than normal or pre-symptomatic HIV-infected brains. A subset of AIDS brains, previously shown to have pleomorphic lymphoid infiltrates in the perivascular spaces, had significantly increased numbers of B lymphocytes in both the brain parenchyma and perivascular spaces. Virtually all AIDS-related PCNSL are known to be Epstein-Barr Virus (EBV) positive, in contrast to non-HIV PCNSL and non-CNS AIDS-related lymphomas. We examined the EBV status of brain parenchymal B lymphocytes to investigate whether EBV-positive B lymphocytes are more frequent in HIV-infected brains than normal, thus explaining the propensity for CNS lymphomas in AIDS. In situ hybridization studies showed EBV positivity only in AIDS-related PCNSL cases within the lymphoma deposits. PCR-based studies detected high EBV copy numbers in PCNSL tumour tissue, and low copy numbers in AIDS cases with pleomorphic lymphoid infiltrates. As none of the B lymphocytes in this latter group were EBV positive on in situ hybridization, bearing in mind that this appears to be a prerequisite for PCNSL development, we find no evidence that pleomorphic infiltrates represent a pre-malignant PCNSL state.