The Impact of Anti-HLA Antibodies on Allogeneic Hematopoieticstem Cell Transplantation Outcome after Standard and Reduced –Intensity Conditioning Regimens

Project location: Italy, Rome
Project start date: July 2015 - Project end date: May 2016
Project number: 2015-018
Beneficiary: A.PRO.TI.ON. Onlus


Allogeneic hematopoietic stem cell transplantation (allo-HSCT) is an effective treatment of both congenital and acquired disturbances of haematopoiesis, especially of haematological malignancies.
The selection of the optimal donor is based on high-resolution HLA typing. The MHC (Major Histocompatibility Complex) contains more than 200 genes which are situated on the short arm of chromosome 6 at 6p21.3. It is divided into three main regions: HLA class I (containing HLA-A, B, and C genes), class II (containing HLA-DR, DQ, and DP genes), and class III region. The role of HLA molecules is to present peptides to T cells (both CD4 and CD8 T cells), enabling them to recognize and eliminate “foreign” particles and also to prevent the recognition of “self” as foreign. HLA mismatches may occur at antigenic or allelic level; the first are characterized by amino acid substitutions in both peptide-binding and T-cell recognition regions, whereas the latter are characterized by amino-acid substitution in the peptide binding regions only.
HLA antigens are recognized by immunocompetent T cells, what may lead to graft failure, graft versus host disease (GVHD), and other post-transplant complications as well as to favourable graft versus leukaemia (GVL) effect. HLA molecules bear multiple antigenic epitopes, many of which are the so-called “public” epitopes that are shared among the products of several different HLA alleles, resulting in the apparent cross-reactive groups of antigens (CREGs). These shared epitopes may be responsible for patient's sensitization to multiple HLA antigens, despite a single antigen mismatch only.
The participation of cellular arm of immunological response to HLA antigens is well known, but the role of humoral arm of immunity is also very interesting, especially when we consider the enormous polymorphism of HLA-genes, their immunogenicity and huge heterogeneity of antibodies. Antibodies are glycoproteins that belong to the super-family of immunoglobulins. The basic structural units of antibodies are two heavy chains (α, γ, ε, δ or μ) and two light chains (κ or λ). The type of a heavy chain determines the class of antibody: IgA, IgG, IgM, IgE or IgM. The region of chromosome that encodes the antibody is large and contains several distinct genes. The locus containing heavy chain genes is found on chromosome 14; loci containing κ and λ light chain genes are found on chromosomes 2 and 22, respectively. The enormous diversity of antibodies allows the immune system to recognize an equally wide variety of antigens. It has been known that humans produce about 10 billion different antibodies capable of binding a distinct epitope of an antigen. Such a diversity of antibodies is caused by domain variability, recombination, somatic hyper mutation and affinity maturation, class switching, and affinity designations. Anti-HLA Abs may be present in healthy individuals. The sensitization to MHC antigens may be caused by transfusions, pregnancy, or failed previous grafts. Anti-HLA Abs are more frequently detected in patients with haematological disorders due to their alloimmunization, resulting mainly from common use of transfusions.
The clinical significance of anti-HLA Abs is well known in the field of transfusional medicine. The presence of anti-HLA Abs in patients is one of the major causes of platelet transfusion refractoriness. On the other hand, anti-HLA Abs present in blood products have been shown to be a major cause of transfusion-related acute lung injury (TRALI). The role of anti-HLA Abs is also well known in solid organ transplantation—especially in kidney transplantation, because transplanted kidneys are highly susceptible to antibody-mediated injury. Antibodies produced before kidney transplantation (reacting with donor's HLA antigens) induce hyper acute or acute vascular rejections which frequently result in transplant failure.
Despite the well-recognized role of antibody-mediated rejection in solid organ transplantation, the graft rejection following allo-HSCT is generally attributed to cytolitic host-versus-graft (HVG) reaction mediated by host T and NK cells, that survived the conditioning regimen. Engraftment failure rate is approximately 4% in allo-HSCT from matched unrelated donor (MUD) and about 20% in cord blood or T-cell-depleted haploidentical transplantations. Antibody-mediated bone marrow failure after allogeneic bone marrow transplantation can be also caused by antibody-dependent cell-mediated cytotoxicity (ADCC), or complement-mediated cytotoxicity. In ADCC, the cytotoxic destruction of antibody-coated target cells by host cells is triggered when an antibody bound to the surface of a cell interacts with Fc receptors on NK cells or macrophages. Preformed antibodies present at the time of hematopoietic stem cell infusion are unaffected by standard transplantation conditioning regimens, T- or B-cell immunosuppressive drugs or modulatory strategies given in the pretransplantation period.

The influence of anti-HLA antibodies, including Abs directed against mismatched antigens, on the results of allo-HSCT, especially on graft failure, has been proved in several reports. However, in patients following allo-HSCT, the series of time remote complications may occur. As antibodies appearing in the result of the earlier immunization are detected before transplantation, the question of their presence and specificity after transplant, after the myeloablative conditioning treatment, and during administration of immunosuppressive therapy is open, when the hematopoietic function is taken over by the donor's cells. The first cells to reconstitute (within the first 100 days) after the transplantation are granulocytes, monocytes, macrophages, and NK cells. In contrast, T and B lymphocytes remain severely reduced and their function is impaired from 6 months to 1 year after the transplantation.

The Nando Peretti Foundation has awarded a grant for this project. APROTION will provide the Molecular laboratory of the Haematology department of Catholic University of Sacred Heart of specific equipment necessary to perform ELISA test for monitoring and evaluating the presence and the role of HLA antibodies in patients with onco-hematological diseases submitted to allogeneic stem cells transplantation.

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