Cytotoxic and stimulatory effects of antilymphocyte globulin (ALG) on hematopoiesis

Immunosuppressive therapies in the management of acquired immune-mediated marrow failures

PURPOSE OF REVIEW

Immunosuppression is a key treatment strategy for patients suffering from aplastic anemia or related immune-mediated bone marrow failure syndromes. Several attempts have been performed to improve the standard immunosuppression regimen of horse antithymocyte globulin (h-ATG) and cyclosporine A (CyA).

RECENT FINDINGS

The addition of a third immunosuppression agent to h-ATG + CyA did not result in any improvement. Antilymphocyte agents other than h-ATG have been investigated. A rabbit-ATG preparation, which was known to be more immunosuppressive than h-ATG, resulted in markedly inferior outcome in a large randomized study from the National Institutes of Health. These data seem to be confirmed in additional experiences with rabbit-ATG from other groups. Cyclophosphamide and alemtuzumab have been proven to be biologically active in small studies, but available data suggest inferior outcomes when compared with h-ATG. All these alternative agents result in a more pronounced lymphocyte depletion, suggesting that the actual mechanisms of action of immunosuppressive therapy in aplastic anemia are not fully understood.

SUMMARY

Immunosuppression by h-ATG and CyA remains the standard of care for aplastic anemia patients lacking a low-risk transplant procedure, resulting in a 60-70% response rate. Rabbit-ATG, cyclophosphamide and alemtuzumab demonstrated a biological activity, but resulted in inferior outcome as compared with h-ATG; thus, they are not recommended as front-line therapy of aplastic anemia.

Horse versus rabbit antithymocyte globulin in acquired aplastic anemia

BACKGROUND

In severe acquired aplastic anemia, hematopoietic failure is the result of immune-mediated destruction of bone marrow stem and progenitor cells. Immunosuppressive therapy with antithymocyte globulin (ATG) plus cyclosporine is an effective alternative to stem-cell transplantation and improves blood counts and survival. Although horse ATG is the standard therapy, rabbit ATG is more potent in depleting peripheral-blood lymphocytes and is preferred in other clinical circumstances.

METHODS

From December 2005 through July 2010, we performed a randomized trial comparing these two ATG formulations in conventional regimens. Patients were treated at a single facility. The primary outcome was hematologic response at 6 months, as determined by blood counts. The study was designed to enroll 60 patients each for the rabbit-ATG and horse-ATG groups and was powered to detect a difference of 25 percentage points in the response rate.

RESULTS

A large, unexpected difference was observed in the rate of hematologic response at 6 months in favor of horse ATG (68%; 95% confidence interval [CI], 56 to 80) as compared with rabbit ATG (37%; 95% CI, 24 to 49; P<0.001). Overall survival at 3 years also differed, with a survival rate of 96% (95% CI, 90 to 100) in the horse-ATG group as compared with 76% (95% CI, 61 to 95) in the rabbit-ATG group (P=0.04) when data were censored at the time of stem-cell transplantation, and 94% (95% CI, 88 to 100) as compared with 70% (95% CI, 56 to 86; P=0.008) in the respective groups when stem-cell-transplantation events were not censored.

CONCLUSIONS

In a randomized study, rabbit ATG was inferior to horse ATG as a first treatment for severe aplastic anemia, as indicated by hematologic response and survival. (Funded by the Intramural Research Program of the National Institutes of Health; ClinicalTrials.gov number, NCT00260689.).

Hypoplastic myelodysplastic syndromes can be distinguished from acquired aplastic anaemia by bone marrow stem cell expression of the tumour necrosis factor receptor

It is often difficult to distinguish hypoplastic myelodysplastic syndrome (h-MDS) from acquired aplastic anaemia (AA), because of the considerable clinical, cytological and histological similarities between these two disorders. The distinction between AA and h-MDS is important because there is a higher risk of progression to acute leukaemia in patients with h-MDS compared with AA. Recent studies suggest that tumour necrosis factor-alpha (TNF-alpha) plays an important role in the development of AA. In order to determine the potential importance of TNF-alpha in the differential diagnosis of hypoplastic bone marrow (BM) disorders, we examined whether analysis ofTNF-receptor expression could be used as a tool to differentiate AA from h-MDS. Flow cytometric analysis revealed that BM stem cells (CD34+) from AA patients have markedly greater TNF receptor (R) 1 and TNFR2 expression than those from patients with MDS and h-MDS. We suggest that the BM stem cells with a high expression of TNFR in patients with AA may be potently sensitive to TNF-alpha stimulation of differentiation. Thus, we propose that quantification of TNFR expression in BM stem cellsmay be a useful method to distinguish AA from h-MDS.

Comparable TNF-alpha, IFN-gamma and GM-CSF production by purified normal marrow CD3 cells in response to horse anti-lymphocyte and rabbit antithymocyte globulin

In vitro priming of T cell with horse antilymphocyte globulin (HALG) results in cytokine release, and this has been associated with its clinical efficacy in patients with severe aplastic anaemia (SAA). Rabbit antithymocyte globulin (RATG) has been studied less extensively. In this study we compare the in vitro priming effect of HALG and RATG on purified normal marrow T cells: end-points of the study were 1) levels of TNF-alpha (TNF-alpha), IFN-gamma (IFN-gamma) GM-CSF in T cell supernatants, and 2) effect of T cell supernatants on colony formation with or without exogenous GM-CSF TNF-alpha, IFN-gamma and GM-CSF levels were comparable for HALG, RATG and phytohaemagglutinin (PHA). T cell supernatants showed comparable enhancement of colony formation in the presence of recombinant human GM-CSF (rhGM-CSF) and supported colony forming unit granulomacrophage (CFU-GM) growth in the absence of growth factor. This study shows that horse and rabbit derived ALG/ATG and PHA have a comparable in vitro priming effect on T cells: both agents should probably be tested for their clinical efficacy in SAA patients.

Gender and response to antilymphocyte globulin (ALG) for severe aplastic anaemia

We have evaluated the speed of haematological recovery in 103 severe aplastic anaemia (SAA) patients treated with antilymphocyte globulin (ALG) and followed at our institution for 3-15 years. We found that haemopoietic recovery was significantly delayed in six girls under the age of 10 years. This slow recovery in girls might be explained by their relative inability to release haemopoietic growth factors, granulocyte colony stimulating activity and burst promoting activity, compared to all other sex and age groups. This defect is not explained by disease severity at presentation and thus indicates a functional abnormality of monocytes/macrophages and T-lymphocytes in addition to the deficiency of haemopoietic stem cells. In a multivariate analysis, low factor production and low pretreatment reticulocyte counts turned out to be strong predictors of slow haemopoietic recovery. We conclude that young girls have a particular form of SAA characterized by low haemopoietic factor production and delayed recovery after ALG. They are preferential candidates for early bone marrow transplantation or, if they are not eligible, for treatment with recombinant human haemopoietic growth factors.

Suspected distinct activation pathways of human lymphocytes induced by antilymphocyte globulin and anti-CD3 monoclonal antibody result in different secretion of hematopoietic colony-stimulating activities

We evaluated the activation sequence of peripheral blood lymphocytes from healthy donors using different mitogens, including antilymphocyte globulin (ALG), anti-CD3 monoclonal antibody (OKT3), and phytohemagglutinin (PHA). Blood mononuclear cells stimulated by ALG, OKT3 and PHA incorporated 3H-thymidine in the same way. When enriched T cells were tested in the presence of interleukin-1 alpha (0 to 100 U/ml, incorporation of 3H-thymidine was greater in those cells stimulated by ALG than by PHA. OKT3 did not activate enriched T cells. Thymidine incorporation was reduced to less than 50% of maximum concentrations by the addition of 10(-7) mol/1,25-dihydroxyvitamin D3 (vit D3) in PHA- or OKT3-activated cells. However, the inhibitory effect of vit D3 was not apparent in ALG-activated cells. Production of granulocyte-macrophage colony-stimulating factor and interleukin-3 by lymphocytes upon activation was consistently higher when cells were treated with ALG or PHA than with OKT3. Taken together, the data indicate that there appear to be distinct functional mechanisms between ALG- and OKT3-induced lymphocyte activation that lead to characteristic immunohematologic events.