Aplastic anemia (AA) is a group of diseases characterized by bone marrow hematopoietic failure and peripheral whole blood cell reduction caused by chemical, physical, and biological factors and unknown causes. The pathogenesis is heterogeneous and is generally believed to be related to intrinsic defects in hematopoietic stem/progenitor cells, defects in hematopoietic microenvironment support functions, and abnormal immune responses that damage hematopoietic stem/progenitor cells, but there is still no more comprehensive elucidation . Now, a growing body of clinical and laboratory evidence suggests that the majority of reblasts develop in association with immune responses, and that immunosuppressive therapy for reblasts is effective, providing more direct evidence for the above arguments. Therefore, the study of the immune mechanism of reoccurrence has become a hot spot of current research and has made great progress, which is reviewed as follows. 1, changes in the number and subpopulation of T lymphocytes Most studies have shown that the proportion of CD8+ T lymphocytes in peripheral blood of patients with reblast increases, the proportion of CD4+ T lymphocytes decreases, and the CD4+/CD8+ ratio decreases; clonal culture of T lymphocytes in bone marrow and peripheral blood of patients with reblast shows that their bone marrow and peripheral blood CD8+ cell clones are significantly higher than those of normal subjects, expressing CD25, HLA-DR, and CD56+ cells were increased [1]. Cytokine abnormalities in remittent disease are highly suggestive of a dominant Th1/Tc1 cell response in remittent disease. the Th1/Th2 ratio is significantly higher in remittent circulating lymphocytes [2]. recognition of antigenic peptides by T lymphocytes is MHC-restricted, HLA-DR2 is associated with morbidity, and the rate of HLA-DR2 positivity is significantly higher in AA patients than in the normal population [3]. remittent patients expressing HLA-DR2 have better efficacy against CSA. 2, T lymphocyte receptor (TCR) expression status TCR belongs to the immunoglobulin superfamily, and the TCR β-chain variable region is encoded by the V, J and D genes. the distal V region, the D region and the proximal J region constitute the CDR3 region of the TCR β-chain variable region gene fragment. Different T cell clones with different lengths or sequences of TCR CDR3 genes form a diverse TCRVβ CDR3 genotype, which determines TCR specificity.Manz et al [4] investigated Vβ1-Vβ 24 transcript expression on the bone marrow and peripheral blood TCRβ chains of AA patients by RT-PCR and found that reentrant patients have a highly Heterogeneous expression pattern of TCRVβ genes existed in patients with retrolisthesis, except for Vβ10, Vβ11, Vβ12, Vβ18, Vβ19, Vβ20, and Vβ22 genes, the rest of the families had different intensities of Vβ gene expression, and the intensity of the expression products was generally higher than that of the controls, with certain subfamilies showing at least a 3-fold increase in expression in AA patients compared with normal controls.The results suggest from the molecular biology AA patients have proliferation of oligoclonal T lymphocytes that are specific and target certain specific antigens. In contrast, after labeling fluorescein with VβPCR products and performing fragment length scans separately, normal controls mostly showed a uniform multi-peak pattern, and the T cell profile was randomly diverse, and there was no dominant expansion of oligoclonal T cells; whereas the CDR3 length peak pattern of Vβ3, Vβ8, and Vβ23 products of patients was the main peak pattern, suggesting that the T cell clones of Vβ3, Vβ8, and Vβ23 genes were taken Lu et al. studied the TCRVβ subfamily of intrahepatic lymphocytes in hepatitis-associated remittent disease, and six of the seven cases tested showed a significant skewed distribution of Vβ21, the degree of which was similar to that of patients with viral hepatitis, while their peripheral blood lymphocytes had a skewed distribution of TCR Vβ The degree of skewed distribution of peripheral blood TCR Vβ was significantly higher than that of healthy controls and tended to be normal after immunosuppressive treatment. The polymorphism of TCR in the bone marrow of AA patients was analyzed, and the presence of oligoclonal lymphocytes abnormally expanded in the bone marrow due to antigenic stimulation was also found, and their TCR expression was analyzed, and the majority of CD4 clones were found to express Vβ5, while CD8 clones expressed Vβ13, and the sequence of the complementary determining region CDR3 was detected in most CD4 clones, but not in normal controls. The CD4 clone exhibited a Th cell secretory manner, lysed autologous CD34 cells and inhibited hematopoietic clone formation. The significant decrease in T cells with CDR3 βV5 clone after immunosuppressive treatment further supports the possibility that T lymphocytes in some patients with remittent disease may be activated by an antigen and proliferate and recognize hematopoietic cells with that antigen expression and the autoimmune response that occurs leads to bone marrow failure. CD28 is one of the most important co-stimulatory molecules constitutively expressed on the surface of T cells, T lymphocytes stimulated by TCR alone cannot be fully activated and can lead to inactivation or programmed death, but simultaneous activation of CD28 can fully activate T lymphocytes; CD28 and CD80 (B7-1) and CD86 (B7-2) on APC are mutual ligand receptors, and their binding can greatly enhance the ability of CD3/TCR complex to activate T cells, and enhance the function of T cells to secrete cytokines, induce the secretion of IL-2, IL-4, IL-5, GM-CSF and other cytokines, and regulate differentiation and proliferative capacity of Th1/Th2 cells, as well as cell cycle progression and susceptibility to apoptosis [8]. The expression of peripheral blood lymphocyte co-stimulatory signal CD28 and its ligands CD80 and CD86 was detected in AA patients, and the results showed that peripheral blood CD4+/CD28+ and CD8+/CD28-T cells were significantly elevated in SAA patients compared with normal controls; CD8+/CD28-T cell expression was decreased in those with effective immunosuppressive therapy; peripheral blood CD8+/CD28 -T cells were elevated compared to normal controls [9].CD28/B7 co-stimulatory signaling increases IL-2 transcription and its mRNA stability, which is a key growth factor for T cells, and increased IL-2 secretion drives CD4+ cell differentiation to the Th1 phenotype.CD4+/CD28+ T cell activation promotes Th1 by promoting IL-2 secretion, promoting Th1 phenotype differentiation and promoting Th1 factor secretion, thus playing a role in the pathogenesis of AA. 4, Apoptotic state of T lymphocytes in AA patients The binding of Fas to its ligand induces the activation of a series of apoptotic effector molecules in cells, which mediates apoptosis by the following pathways: Kim et al. found that due to defects in the Fas system leading to activation-induced cell death mediated by Fas and FasL interactions, the AICD is abnormal and the number of activated lymphocyte deaths is reduced, participating in the AA Brazil et al. found that Fas antigen was expressed in cells of both patients with remittent disease and normal subjects, but the expression of Fas antigen CD95 was significantly higher in CD34+ cells of patients with remittent disease than in normal subjects. T lymphocytes in patients with remitting disease are highly secreting IFN- γ and TNF- α, etc. These two can induce high expression of Fas antigen in CD34+ cells, and induce massive apoptosis of bone marrow CD34 ten cells through the combination of Fas and FasL, which leads to bone marrow hematopoietic failure. Foreign scholars measured Fas receptors (FasR) in peripheral blood and bone marrow CD3 and CD19 lymphocytes from control, AA, AA in complete remission, and multiple transfusion but not AA patients and found that FasR was overexpressed in peripheral blood and bone marrow T and B lymphocytes from AA patients, but no such abnormality was found in AA complete remitters or multiple transfusion patients with other hematologic disorders.AA patients with CD3 /FasR cells were not increased and the crosslinking of FasR in their lymphocytes caused an increase in apoptotic messages, but the crosslinking of FasR in lymphocytes from normal controls, AA patients in complete remission, or multiple transfusions but not AA patients did not cause an increase in apoptotic messages. It is believed that overexpression of Fas on the surface of CD34+ cells, T and B lymphocytes in AA patients is characteristic of AA, and apoptosis in AA patients is mainly through Fas and FasL. Liu et al. used PCR to design a fragment of siRNA that could be expressed in plasmids, and this fragment effectively blocked the expression of Fas in mammalian cells, resulting in a decrease in apoptosis, and this experiment This experiment lays the foundation for further research on the use of Fas inhibitors in the treatment of diseases such as remittent disease. 5. The role of cytokine abnormalities in the pathogenesis of AA AA patients exhibit significant inhibitory activity due to changes in T lymphocyte subsets and function and dysregulation of lymphokine secretion, which can produce excessive amounts of hematopoietic negative regulatory factors. Studies have shown that bone marrow T lymphocytes from AA patients can spontaneously produce hematopoietic negative regulatory factors such as IFN-γ , IL-2, TNF-α, macrophage inflammatory protein (MIP)-1α, and Skil in vitro without pre-stimulation. 5.1 Interleukin-2 (IL-2) IL-2 is mainly produced by activated T cells, which is necessary for T cell proliferation and can induce and enhance the effect of NK cells and CTL cells. The level of IL-2 in the serum and supernatant of mononuclear cell culture of patients with reblindness is elevated, and some patients’ mononuclear cells can spontaneously produce IL-2, indicating that IL-2 abnormalities are associated with hematopoietic suppression in reblindness. Recently, some scholars have achieved good results in treating patients with mild remittent disease with anti-IL-2 receptor monoclonal antibodies, treating 16 patients with 6 therapeutic responses, including 2 cases with complete recovery of blood picture, 4 cases with a series of elevated blood picture, 2 transfusion-dependent patients who could stop transfusion, and 1 patient with neutropenic recurrent infections with normal neutrophils; and recently Sloang et al. used anti-IL-2 receptor antibody (daclizumab) to treat pure red blood cell anemia, 6 of 15 patients (40%) responded to treatment within 90 days, while all patients returned to normal hemoglobin levels after 18 months of treatment [14], which in turn provides a direct basis for the involvement of IL-2 in the hematopoietic suppression process of reperfusion. 5.2 IFN-γ and TNF-αIFN-γ is a type ΙΙ interferon that inhibits granulocyte, erythroid, megakaryocyte lineages, pluripotent stem cells, and B cells.One of the main mechanisms by which IFN-γ inhibits hematopoiesis is by triggering the apoptotic system, and after several days of incubation of IFN-γ and/or TNF-α with hematopoietic cells, apoptosis occurs throughout the bone marrow cells and CD34+ cells. Gel electrophoresis showed typical DNA degradation changes in “ladder” DNA and in situ terminal deoxyribonucleotidyl transferase assay, which resulted in a large number of apoptotic cells due to the upregulation of Fas receptor expression on the cell membrane surface by IFN-γ. When the IFN-γ gene was introduced into normal bone marrow stromal cells and they were continuously secreting low concentrations of IFN-γ, the yield of BFU-E and CFU-GM was lower than that of the control group after 5 weeks; and the additional IFN-γ concentration could not inhibit CD34+ cells, and to achieve the same inhibitory effect, the additional IFN-γ concentration would need to be 100 times higher than the endogenous IFN-γ concentration. or more. These facts suggest that T cells inhibit bone marrow cell growth through their secreted cytokines: high local concentrations of inhibitory cytokines in the bone marrow are associated with the development of AA, whereas very low levels of hematopoietic inhibitory factors can have a significant effect when acting in close proximity to hematopoietic stem progenitor cells. IFN-γ stimulates T-cell proliferation in transgenic mice, and the proliferating T-cells in turn secrete IFN-γ, creating a negative cycle that inhibits hematopoiesis. IFN-γ activates many signaling pathways in target cells, and IFN-γ and TNF-α induce NO synthetase expression in CD34+ cells, which is toxic to CD34+ cells; IFN-γ can also inhibit hematopoiesis by inducing transcription factor-IFN regulatory factor-1. In vitro experiments have shown that IFN-γ and TNF-α can inhibit the proliferation of hematopoietic stem/progenitor cells in tissue culture, while TNF-α can coordinate the effect of IFN-γ. The addition of anti-TNF-α antibody to bone marrow cells cultured in vitro can increase the size and number of BFU-E and CFU-E. 6. Bone marrow MSCs and AA Bone marrow MSCs (MSCs) can differentiate into osteoblasts, chondrocytes, adipocytes, and vascular endothelial cells, and studies have suggested that MSCs have immunosuppressive effects both in vivo and ex vivo [19]. In experiments with mouse models in which the major histocompatibility antigens (MHC) were mostly incompatible, after administration of lethal doses of radiation, these mice either failed bone marrow transplantation or died of graft-versus-host disease; however, if an equal amount of donor osteoblasts were added to the donor bone marrow, 100% of the bone marrow grafts were successfully grown in the above-mentioned mouse models, including regeneration of blood cells and recovery in terms of immune mechanisms MSCs are also immunosuppressive in vitro, and their addition in experiments resulted in a decrease in T-lymphocyte activity. Recently, it was found that the interaction between MSC and dendritic cells, NK cells and T cells resulted in the production of prostaglandin E2, which causes the immune system to upregulate the amount of IL-4 and IL-10 and downregulate the amount of INF and TNF. Bartholomew collected 19 healthy bone marrow from 23 patients (3 of whom were newly diagnosed with repletion, 16 of whom were treated with immunosuppressive therapy, and 13 of whom The MSCs were isolated and cultured in vitro, and after three passages, the surface markers of MSCs from patients with recurrent disease and normal control cells were identified by flow cytometry. They added phytohemagglutinin to the T cell line, and 72 hours later added MSC from cultured normal controls, and found that T lymphocyte proliferation induced by phytohemagglutinin was inhibited and positively correlated with the amount of MSC added; whereas the ability to inhibit T lymphocyte proliferation was greatly diminished by adding MSC from patients with remittent disease to T lymphocytes, but the proliferation of T lymphocytes by MSC from bone marrow transplant patients inhibitory effect was restored. The amount of γ-IFN secreted by T lymphocytes in the supernatant of both was detected, and the amount of γ-IFN produced by T lymphocytes added to MSC from normal controls was significantly reduced, while the amount of γ-IFN secreted by T cells added to MSC from replete patients was not reduced, and the amount of γ-IFN from patients who underwent bone marrow transplantation was reduced; the expression of T cell CD38 was also detected, and T cells added to phytohemagglutinin and co-cultured with The expression of T-cell CD38 was significantly reduced when T cells were co-cultured with phytohemagglutinin and normal MSC, but was barely affected in the co-culture system of T cells with MSC from patients with remittent disease (whether immunotherapy was effective or not). Thus, Bartholomew et al. hypothesized that normal MSC could release 2,3-dioxidase indoleamine (IDO), which is the main substance that inhibits T-cell activity, while MSC from patients with reoccurrence lacked the ability to inhibit T-cell proliferation and cytokine release, and therefore, whether they were newly diagnosed with reoccurrence, treated with immunosuppressive therapy, treated with effective immunotherapy, relapsed with transfusion, or The ability to suppress T-cell proliferation and activity remained low even after several years of immunosuppressive therapy. Based on the above findings, we believe that reoccurrence is an autoimmune disease and its pathogenesis is the result of the interaction of multiple immunological mechanisms. A large number of phenomena and pathogenesis have not been elucidated during the study: for example, is the increase of activated T lymphocytes in AA patients related to the failure of timely and effective apoptosis of T lymphocytes? Is the expression of CD28 abnormal in AA patients, and how does CD28 expression change in vivo and after in vitro application of ATG+CSA and what are its possible mechanisms? Can the results of in vitro culture of bone marrow MSCs from regrowth patients be equated with the in vivo bone marrow microenvironment, after all, the in vivo environment is the result of a combination of factors, and the exact mechanism of its suppression of T cell activity and proliferative capacity is not yet clear. In-depth studies on the abnormal manifestations, mechanisms and causes of T lymphocytes and their subtypes in AA patients, and the effects of bone marrow MSCs on hematopoietic stem cells in AA patients are important to systematically elucidate the immunopathological mechanisms of AA and thus guide clinical treatment.