Human bone marrow as a source to generate CMV-specific CD4+ T cells with multifunctional capacity.

The bone marrow (BM) is an important compartment for T cell memory. In cytomegalovirus (CMV)-seropositive individuals peripheral blood (PB) CMV-specific T cells constitute a large fraction of PB T cells but are mostly differentiated effector/effector memory T cells with limited survival and proliferative potential. In this study, we performed a comprehensive analysis of the CMV-specific T cell response in BM studying both CD4+ and CD8+ T cell responses against overlapping peptide pools of the CMV proteins pp65 and immediate early protein-1. CMV-specific T cell responses were characterized ex vivo and after in vitro expansion of paired PB/BM samples by multiparameter flow cytometry determining surface phenotype, cytokine profile, and cytotoxic capability. Comparable frequencies of CMV-specific T cells were found in un-manipulated PB and BM. Both total CD4+ and CD8+ T cells could be more rapidly expanded from BM. Expanded BM T cells contained significantly higher frequencies of CMV-specific CD4+ T cells than PB. Furthermore, higher frequencies of specific CD4+ T cells from BM were multifunctional, characterized by simultaneous production of interferon-gamma, tumor necrosis factor, and interleukin-2. Use of BM may thus facilitate more rapid generation of adoptive T cells with enhanced functionality.

CMV-specific central memory T cells reside in bone marrow.

CMV-specific CD8(+) T cell responses in peripheral blood (PB) are characterized by a preponderance of effector and effector memory T cells. CMV-specific central memory T cells (T(CM)), which are considered crucial in maintaining long-term immunity, are rarely detectable in PB. In this study we have analyzed differentiation and function of CMV pp65-specific CD8(+) T cells in paired samples of human PB and BM using intracellular cytokine and tetramer staining. Overall frequencies of CMV pp65-specific T cells were similar in PB compared to BM; however, CMV-specific CD45RA(-)CCR7(+) T(CM) were almost exclusively detectable in BM, which was not related to a general accumulation of T(CM) in BM. In vitro, CMV-specific T cells could be more efficiently expanded from BM (median 128-fold, n=6) than from PB (median 72-fold, p=0.01). Taken together, these data show that the BM is a compartment harboring CMV-specific T(CM) and underline the concept of the BM as a secondary immune organ. CMV specific BM-derived T(CM) might be a valuable source for generating T cells for adoptive transfer.