Tumor infiltrating B cells as antigen presenting cells: their role in mediating the anti-tumoral immune response induced by a combined immune-stimulatory/conditional cytotoxic approach for glioblastoma

MARIANELA CANDOLFI; JAMES F CURTIN; KADER YAGIZ; HIKMAT ASSI; KURT M KROEGER; MATTHEW TESARFREUND; ANDREW ALDEN; CHUNYAN LIU; ROBERT DUNN; PEDRO R LOWENSTEIN; MARIA G CASTRO

Denver, CO, EEUU

Congreso; American Association for Cancer Research 100 Annual Meeting; 2009

Although B cells are well known by their role as producers of Ag specific immunoglobulins, they can also act as efficient antigen presenting cells (APCs). The clonal expression of BcR and the high specific avidity for Ag allow B cells to accumulate and concentrate low density extracellular Ag for presentation to T cells, lowering the threshold of Ag density required for triggering adaptive immune responses. The role of B cells as APCs mediating brain tumor regression in response to immunotherapy is not clearly understood. We have developed adenoviral vectors (Ads) expressing TK (Ad-TK; a conditional cytotoxic molecule) and Flt3L (Ad-Flt3L; an immune-stimulatory cytokine), which increase the survival of rodent models of GBM. CD4+ and CD8+ T cells are necessary for Ad-TK+Ad-Flt3L-mediated tumor regression and immunological memory. Here we investigated whether B cells also play a role in GBM regression following Ad-TK+Ad-Flt3L. While Ad-TK+Ad-Flt3L induced tumor regression and long term survival in 60% of the wild type (WT) mice, the treatment completely failed in B-cell deficient Igh6-/- mice. Similarly, in WT mice depleted of total B cells (using anti-CD20 Abs), or marginal zone B cells (MZB, using anti-CD11a and anti-CD49d Abs), treatment with Ad-TK+Ad-Flt3L failed. Spleen CD19+ B cells did not expand, while the number of MZB cells increased at day 7 post-treatment. We did not find changes in the number of B cells within the tumor mass or draining lymph nodes, and we could not detect circulating Abs against tumor cells. We then studied whether B cells could be acting as tumor antigen presenting cells during the onset of the anti-tumoral immune response induced by Ad-TK+Ad-Flt3L. We found that B cells purified from the draining lymph nodes of Ad-TK+Ad-Flt3L-treated, but not from control tumor bearing C57/B6 mice significantly increased the proliferation of T cells purified from spleen of naïve Balb/c mice. Also, tumor-specific T cell clonal expansion was studied in an ELISPOT using splenocytes from tumor-bearing WT or Igh6-/- mice treated with saline or Ad-TK+Ad-Flt3L, and stimulated with Trp2180-188, a MHCI-associated GL26 tumor antigen. Trp2-specific T cell precursors were detected only in Ad-TK+Ad-Flt3L-treated WT mice, but we did not observe any increase in the frequency of tumor antigen specific T cells in Igh6-/- mice. Together, this data indicates that B cells play an important role in facilitating clonal expansion of tumor antigen specific T cells and subsequent elimination of brain tumors, most likely by acting as antigen presenting cells, rather than antibody-producing cells.