INTRODUCTION: Depletion of T cells by monoclonal antibodies is a widely used therapeutic approach for inducing tolerance and preventing graft versus host disease in patients with organ transplants (1). Although a substantial number of CD4 T cells can escape depletion, there are no data on their specific tissue location and phenotypic characteristics in different peripheral lymphoid tissues (2). In the present study we investigated survival, phenotype and location of CD4-positive T cells immediately following anti-Thy-1 monoclonal antibody treatment in mouse. METHODS: BALB/c mice at 6-10 weeks of age were injected i.v. 300 µg of IBL-1 (anti-Thy-1/CD90) monoclonal antibody on two consecutive days and sacrificed 36 hours after first injection. In a separate set of experiments, prior to IBL-1 treatment, mice were immunized by human RBC (100µl/recipient of a 10% suspension in PBS i.p,) or treated by lymphotoxin beta-receptor fusion protein (100 µg/recipient i.v.), for induction of germinal centre reaction or acute disruption of follicular structure, respectively. For the purpose of evaluating the effect of acute IL-7 withdrawal, mice received simultaneously both anti-Thy-1 (600 µg/recipient) and polyclonal rabbit anti-mouse IL-7 (160 µg/recipient) blocking Abs. Their spleen, lymph nodes and Payers patches (PP) were collected and further processed for the fow cytometric and immunofluorescent microscopic assessment using CD62L, CD44, CD4, CD8, Bcl-2, CXCR5 and PD1 markers. T-cell proliferation was monitored by using BrdU incorporation assay. RESULTS: We found a preferential survival of CD4 T cells in the Peyer’s patches and, to a lesser degree, in the mesenteric lymph nodes of BALB/c mice, where majority of the surviving CD4-positive T cells displayed CD44hi/CD62Lneg phenotype corresponding to effector memory T cell features. In addition, these CD4-positive cells also expressed CXCR5 and PD-1 markers characteristic for follicular TH cells (TFH), and were enriched in the Peyer’s patch follicles. Using BrdU-incorporation we demonstrate that the immediate survival of these cells does not involve proliferation. Furthermore, simultaneous blockade of IL-7 did not enhance the efficiency of anti-Thy-1 depletion. Induction of germinal center formation during T-dependent immune response in spleen resulted in enhanced survival of splenic TFH cells, while the dissolution of follicular architecture by lymphotoxin beta-receptor antagonist treatment slightly reduced TFH survival. CONCLUSION: Our results thus raise the possibility that the natural germinal center microenvironment in the GALT may be favorable for the survival of T-follicular helper cells of memory phenotype.