B Cell Depletion Reduces the Number and Function of T- Regulatory Cells and Enhances the Anti Tumor response

Abstract 3668

Poster Board III-604

Increasing evidence suggests that B lymphocytes play a central role in inhibiting the immune response against certain tumors, but the underlying mechanisms by which B cells facilitate tumor growth are still poorly understood.

In this study, we investigated how the presence or absence of B cells affects expansion and function of T- regulatory cells (‘Tregs’) in a murine tumor model (EMT-6). We compared tumor growth, and the number and function of T- regs cells in wild type immune competent mice ( ICM), B cell deficient mice ( BCDM) and /or in BALB-C mice following B- cell depletion induced by injection of anti murine CD20 antibodies (mCD20 Ab, 18B12, mouse IgG1,k, Biogen-IDEC) Mice were either tumor-naïve or implanted with EMT6 mammary adenocarcinoma cells.

Absence of B cells as in BCDM completely inhibited tumor growth in the majority of mice, while B cell depletion in normal mice substantially slowed the growth of EMT-6 tumors compared to wild type mice (ICM). Substantial T regs expansion, as defined by CD4+/CD25+/FOXP3+ cells, was evident on day 26 post tumor inoculation in EMT-6 tumor bearing ICM in comparison to the non- tumor bearing mice ( 15.2 +/− 1.2. % and 11.9 +/− 1.1% respectively), isolated from spleen as compared to naïve or tumor bearing BCDM (10.1+/− 0.2% and 10.8+/− 1.2%) The percentage and absolute number of T-regs in the spleen, tumor draining lymph nodes and tumor bed were significantly reduced in the BCDM and/or B cell depleted ICM compared to tumor bearing ICM (10%+/−0.8, 13.9+/− 1.23% and 17+/− 1.3% respectively p<0.01. data from single cell suspensions isolated from spleens on day 20 post tumor inoculation). Similar effects of B cell depletion on the numbers of T-regs were observed in the setting of pre-established EMT6 mammary tumors. In contrast to tumor bearing mice, differences in T-reg number and function were minimal in tumor free B cell deficient or in B cell depleted naïve mice compared to ICM.

T-reg function, measured by suppression assay and proliferation assays, was also markedly reduced in tumor bearing BCDM compared to ICM. Combining B cell and T-reg depletion using i.p. injection of anti CD 25 antibody (PC61 or PBS) resulted in similar rates of tumor regression in B cell depleted mice as were seen in BCDM suggesting that the combination of B cells depletion and further depletion of Tregs augmented anti-tumor response.

In conclusion, our studies indicate that B cell depletion may play a useful role in augmenting the T cell anti-tumor response, in part due to their effects on T-regulatory cell biology.