Bone marrow T cells from the femur are similar to iliac crest derived cells in old age and represent a useful tool for studying the aged immune system

Bone marrow expands the repertoire of functional T cells targeting tumor-associated antigens in patients with resectable non-small-cell lung cancer

The efficacy of cancer immunotherapy may be improved by increasing the number of circulating tumor-reactive T cells. The bone marrow is a priming site and reservoir for such T cells. The characteristics of bone marrow-derived tumor-reactive T cells are poorly understood in patients with non-small-cell lung cancer (NSCLC). To compare the responsiveness of tumor antigen-reactive T cells from the bone marrow with matched peripheral blood samples in patients with resectable NSCLC, we used flow cytometry, cytokine capture assays and enzyme-linked immunospot assays to examine the responsiveness of T cells to 14 tumor antigens in matched bone marrow and peripheral blood samples from patients with resectable NSCLC or benign tumors and tumor-free patients. T cells with reactivity to tumor antigens were detected in the bone marrow of 20 of 39 (51%) NSCLC patients. The panel of tumor antigens recognized by bone marrow-derived T cells was distinct from that recognized by peripheral blood-derived T cells in NSCLC patients. Unlike for peripheral blood T cells, the presence of tumor-reactive T cells in the bone marrow did not correlate with recurrence-free survival after curative intent resection of NSCLC. T cells with reactivity to tumor antigens are common in the bone marrow of patients with NSCLC. Tumor-reactive T cells of the bone marrow have the potential to significantly broaden the total repertoire of tumor-reactive T cells in the body. To clarify the role of tumor-reactive T cells of the bone marrow in T cell-based immunotherapy approaches, clinical studies are needed (ClinicalTrials.gov: NCT02515760).

Peripheral and systemic antigens elicit an expandable pool of resident memory CD8+ T cells in the bone marrow

BM has been put forward as a major reservoir for memory CD8⁺  T cells. In order to fulfill that function, BM should "store" memory CD8⁺ T cells, which in biological terms would require these "stored" memory cells to be in disequilibrium with the circulatory pool. This issue is a matter of ongoing debate. Here, we unequivocally demonstrate that murine and human BM harbors a population of tissue-resident memory CD8⁺ T (T_RM ) cells. These cells develop against various pathogens, independently of BM infection or local antigen recognition. BM CD8⁺ T_RM cells share a transcriptional program with resident lymphoid cells in other tissues; they are polyfunctional cytokine producers and dependent on IL-15, Blimp-1, and Hobit. CD8⁺ T_RM cells reside in the BM parenchyma, but are in close contact with the circulation. Moreover, this pool of resident T cells is not size-restricted and expands upon peripheral antigenic re-challenge. This works extends the role of the BM in the maintenance of CD8⁺ T cell memory to include the preservation of an expandable reservoir of functional, non-recirculating memory CD8⁺ T cells, which develop in response to a large variety of peripheral antigens.

T cells in multiple myeloma display features of exhaustion and senescence at the tumor site

BACKGROUND

Multiple myeloma is an incurable plasma cell malignancy that is mostly restricted to the bone marrow. Cancer-induced dysfunction of cytotoxic T cells at the tumor site may be responsible for immune evasion and therapeutical failure of immunotherapies. Therefore, enhanced knowledge about the actual status of T cells in myeloma bone marrow is urgently needed. Here, we assessed the expression of inhibitory molecules PD-1, CTLA-4, 2B4, CD160, senescence marker CD57, and CD28 on T cells of naive and treated myeloma patients in the bone marrow and peripheral blood and collected data on T cell subset distribution in both compartments. In addition, T cell function concerning proliferation and expression of T-bet, IL-2, IFNγ, and CD107a was investigated after in vitro stimulation by CD3/CD28. Finally, data was compared to healthy, age-matched donor T cells from both compartments.

METHODS

Multicolor flow cytometry was utilized for the analyses of surface molecules, intracellular staining of cytokines was also performed by flow cytometry, and proliferation was assessed by ³H-thymidine incorporation. Statistical analyses were performed utilizing unpaired T test and Mann-Whitney U test.

RESULTS

We observed enhanced T cell exhaustion and senescence especially at the tumor site. CD8+ T cells expressed several molecules associated with T cell exhaustion (PD-1, CTLA-4, 2B4, CD160) and T cell senescence (CD57, lack of CD28). This phenotype was associated with lower proliferative capacity and impaired function. Despite a high expression of the transcription factor T-bet, CD8+ T cells from the tumor site failed to produce IFNγ after CD3/CD28 in vitro restimulation and displayed a reduced ability to degranulate in response to T cell stimuli. Notably, the percentage of senescent CD57+CD28- CD8+ T cells was significantly lower in treated myeloma patients when compared to untreated patients.

CONCLUSIONS

T cells from the bone marrow of myeloma patients were more severely impaired than peripheral T cells. While our data suggest that terminally differentiated cells are preferentially deleted by therapy, immune-checkpoint molecules were still present on T cells supporting the potential of checkpoint inhibitors to reactivate T cells in myeloma patients in combination therapies. However, additional avenues to restore anti-myeloma T cell responses are urgently needed.

Quantitative and Qualitative Analysis of Bone Marrow CD8(+) T Cells from Different Bones Uncovers a Major Contribution of the Bone Marrow in the Vertebrae

Bone marrow (BM) plays an important role in the long-term maintenance of memory T cells. Yet, BM is found in numerous bones throughout the body, which are not equal in structure, as they differ in their ratio of cortical and trabecular bone. This implies that BM cells within different bones are subjected to different microenvironments, possibly leading to differences in their frequencies and function. To address this, we examined BM from murine tibia, femur, pelvis, sternum, radius, humerus, calvarium, and the vertebrae and analyzed the presence of effector memory (TEM), central memory (TCM), and naïve (TNV) CD8(+) T cells. During steady-state conditions, the frequency of the total CD8(+) T cell population was comparable between all bones. Interestingly, most CD8(+) T cells were located in the vertebrae, as it contained the highest amount of BM cells. Furthermore, the frequencies of TEM, TCM, and TNV cells were similar between all bones, with a majority of TNV cells. Additionally, CD8(+) T cells collected from different bones similarly expressed the key survival receptors IL-7Rα and IL-15Rβ. We also examined BM for memory CD8(+) T cells with a tissue-resident memory phenotype and observed that approximately half of all TEM cells expressed the retention marker CD69. Remarkably, in the memory phase of acute infection with the lymphocytic choriomeningitis virus (LCMV), we found a massive compositional change in the BM CD8(+) T cell population, as the TEM cells became the dominant subset at the cost of TNV cells. Analysis of Ki-67 expression established that these TEM cells were in a quiescent state. Finally, we detected higher frequencies of LCMV-specific CD8(+) T cells in BM compared to spleen and found that BM in its entirety contained fivefold more LCMV-specific CD8(+) T cells. In conclusion, although infection with LCMV caused a dramatic change in the BM CD8(+) T cell population, this did not result in noticeable differences between BM collected from different bones. Our findings suggest that in respect to CD8(+) T cells, BM harvested from a single bone is a fair reflection of the rest of the BM present in the murine body.

Plasma cell numbers decrease in bone marrow of old patients

The BM is well understood to play a key role in plasma cell homing and survival in mice. In humans, BM plasma cells and their functions are less well characterized. In this study, we used paired bone biopsies from the femur shaft and blood samples from persons of different ages to analyze age-related changes of plasma and memory B cells. Our results demonstrated that plasma cells were mainly located in the BM, while a higher percentage of memory B cells was in the peripheral blood than in the BM. The frequency of plasma and memory B cells from both sources decreased with age, while immature and naïve B cells were unaffected. An age-related decline of tetanus- and diphtheria-specific BM plasma cells was observed, whereas influenza A- and cytomegalovirus-specific BM plasma cells were not affected. With the exception of cytomegalovirus, peripheral antibody concentrations correlated with BM plasma cells of the same specificity, but were independent of antigen-specific peripheral blood memory B cells. Our results demonstrate that the BM houses decreased numbers of plasma cells in old age. The number of cells of certain specificity may reflect the number and time point of previous antigen encounters and intrinsic age-related changes in the BM.

Regulation of myelopoiesis by CD137L signaling

CD137 ligand (CD137L) has emerged as a powerful regulator of myelopoiesis that links emergency situations, such as infections, to the generation of additional myeloid cells, and to their activation and maturation. CD137L is expressed on the cell surface of hematopoietic stem and progenitor cells (HSPC) and antigen presenting cells (APC) as a transmembrane protein. The signaling of CD137L into HSPC induces their proliferation and differentiation to monocytes and macrophages, and in monocytes CD137L signaling induces differentiation to potent dendritic cells (DC). CD137L signaling is initiated by CD137 which is expressed by T cells, once they become activated. Some of these activated, CD137-expressing T cells migrate from the site of infection to the bone marrow where they interact with HSPC to induce myelopoiesis, or they induce monocyte to DC differentiation locally at the site of infection. Therapeutically, induction of CD137L signaling can be utilized to reinitiate myeloid differentiation in acute myeloid leukemia cells, and to generate potent DC for immunotherapy.