Memory T cells skew toward terminal differentiation in the CD8+ T cell population in patients with acute myeloid leukemia

Clinical applications of immunotherapy for AML lag behind those for solid tumors and lymphocytic leukemia [1‐3]. Recently, a new memory T cell subset, stem cell memory T (T_SCM), which has stem cell-like capacity, has been discovered [4‐6]. However, little is known about the role of these cells in AML. In this study, we assessed the distribution of CD4+ and CD8+ T_SCM, central memory T (T_CM), T effector memory (T_EM), and T terminal effector (T_EF) cells in peripheral blood (PB) and bone marrow (BM) from patients with AML and those with AML in complete remission (AML-CR) by multicolor flow cytometry. The gating strategy used in this study followed a published protocol [7]. The CD4+ and CD8+ T cells were divided into four subgroups according to the CCR7 and CD45RO expression pattern: naïve and T_SCM cells (CCR7+CD45RO−), T_CM cells (CCR7+CD45RO+), T_EM cells (CCR7−CD45RO+), and T_EF cells (CCR7−CD45RO−). The T_SCM population was defined by double positive CD95 and CD28 expression.

The percentages of the T_SCM, T_CM, T_EM, and T_EF cells in the CD4+ and CD8+ populations were analyzed in 20 cases with AML (17 cases in newly diagnosed and 3 cases with AML relapse) (Fig. 1a, d) [8, 9]. The CD8+ T_SCM and CD8+ T_CM cells significantly decreased in the PB of these patients (Fig. 1e, g), whereas there was no significant change in the CD4+ population (Fig. 1b, g). Thus, the changes in the memory T cell subsets appeared to mainly involve CD8+ T cells. The shift from T_SCM and T_CM cells to a higher ratio of differentiated T_EM and T_EF cells is thought to be due to the constant exposure of T cells to AML cells and the leukemia environment, leading to T cell exhaustion and/or dysfunction [3].

To study the influence of the tumor microenvironment on the memory T cell distribution and function in leukemia patients, we collected seven pairs of PB and BM samples from AML patients at the time of diagnosis and compared the distributions of memory T cell subsets. The differences in each subset appeared to vary widely (Fig. 1c, f). A low percentage of CD4+ T_CM cells and a corresponding high percentage of CD4+ T_EM and T_EF cells were observed in the BM compared with PB (Fig. 1c). In the CD8+ population, the changes appeared to be specific to each individual, and lower CD8+ T_SCM and CD8+ T_CM percentages were observed in the BM in half of the patients, whereas there were high percentages of CD8+ T_SCM and CD8+ T_CM cells in the BM compared with PB in the remaining samples. It has been reported that T cells in normal BM mainly possess a memory phenotype, particularly for CD8+ T_CM cells [10], suggesting that alterations in the leukemic BM niche in different AML individuals and AML subtypes may have different impact on T_CM homing.

Next, we compared the distribution of memory T cells in AML patients younger (AMLy) and older (AMLo) than 60 years [11]. Unlike healthy individuals (HIs), the memory T cell subset distribution in the AMLy cohort was strikingly different than that in younger HIs (HIy) and tended to have a similar distribution pattern as that detected in the HIo and AMLo groups with a more obvious difference in the CD8+ population (Figs. 1g and 2a, b). These findings indicate that the leukemia microenvironment might drive T cell differentiation in AMLy. Whether such a skewed T cell distribution in AMLy truly represents T cell senescence remains an open question [8]; however, T cells in AMLo patients may not be able to further differentiate due to inherent T cell senescence, which may be an immune factor underlying the inferior prognosis of AMLo patients. Together, these data may suggest that T cell exhaustion and senescence are involved in T cell immune impairment, leading to an inefficient anti-tumor response.

We next compared differences in the distribution of memory T cell subsets between the AMLy, AML-CR, and HIy groups. A persistent, skewed memory T cell distribution was demonstrated for AML patients who achieved CR after chemotherapy (Fig. 2c, d). CD4+ and CD8+ T_SCM cells were predominantly increased at different time points after CR, while the change in other memory T cell subsets was relatively different (Fig. 2e, f). Overall, with the exception of incomplete recovery of the T_SCM cells, the reduction in T_CM cells and corresponding excessive accumulation of T_EM and T_EF cells were more evident in AML patients with CR (Fig. 1g), which may be related to the immune suppression of chemotherapy.