Introduction. The aim of this study was to evaluate the role of host immune system, particularly circulating CD4⁺CD25⁺FoxP3^high cells, in the growth regulation of tumours in murine DBA/2-SL2 lymphoma model.

Materials and methods. Murine immune system was suppressed by the intraperitonial injection of high dose cyclophosphamide (CTX). The volume of two SL2 tumours implanted in the same mouse was analysed in CTX pre-treated and control groups. Lymphocytes count was obtained by an automatic haematological analyzer. The frequency of circulating regulatory T cells CD4⁺CD25⁺FoxP3^high was determined in the peripheral blood of mice in each group using regulatory T cells antibody staining kit by flow cytometry.

Results. CTX treatment significantly reduced lymphocyte count (3.84 fold decrease) and CD4⁺CD25⁺FoxP3^high cells percentage (1.83 fold decrease). Primary and secondary tumours in CTX treated group reached significantly smaller volumes than in the control group. Our results indicate that growth of tumours in CTX pre-treated mice is inhibited despite general immunosuppression, contrary to the general observation that occurrence of tumours is increased in immunodeficient / immunosuppressed hosts.

Conclusions. Pre-treatment with CTX decreased tumour volume in murine DBA2-SL2 lymphoma model and this effect can be attributed to systemic CTX effect on murine immune system. In our study we exclude the direct effect of CTX and demonstrate that changes in tumour growth dynamic could be related to the general immunosuppression status and reduced T regs frequency in mice.

Keywords: regulatory T cells, tumor immunology, cyclophosphamide