Regulation of antibody production in immunocompromised patients

Abstract

The aim of this thesis was to study B-cell function and the regulation of specific antibody (Abs) production in immunocompromised individuals. As models, we have studied children with acute lymphoblastic leukaemia (ALL) and patients with IRV-I infection. In the first group, leukaemia by itself causes immunosuppression due to impaired haematopoiesis. In addition, the subsequent treatment with chemotherapy affects the immune system. In the latter group, the viral infection causes immunosuppression and deterioration of immune functions.

Hypergammaglobulinemia is a common finding in autoimmune disease, chronic infections and in lymfoproliferative disorders. The regulation of specific Ab production in HIV and childhood ALL was studied in relation to hypergammaglobulinemia. When analysing anti-measles Alia in these two patient groups an important difference was noted. Specific Ab levels were decreased in HIV-1 infected subjects even in presence of high total immunoglobulinG (IgG) levels whereas the high serum IgG levels in the children with ALL reflected elevated levels of specific Abs. This indicates that the memory B cell compartment in children with ALL is conserved at diagnosis and activated by the disease, Afifle the memory B cell compartment is compromised during HIV-1 infection.

HIV-infected subjects with a reduced memory B-cell pool showed reduced titers of specific Abs compared to HIVinfected subjects with a normal memory B-cell pool and healthy controls. This indicates that long-term immunity is maintained by memory-B-cells. However, hypergammaglobulinemia is present in both groups of HIV subjects irrespectively of the number of memory B-cells. The high total IgG contains high levels of polyspecific Abs in the IRV-infected subjects compared to healthy individuals. The source of the polyspecific, Abs in HIV is naïve CD70+ B-cells containing intracellular IgG and secreting IgG in vitro.

During chemotherapy plasma cells as well as memory B cells are reduced and in approximately 40% of the ALL children the levels of vaccination induced specific Abs have declined. Memory B-cells and plasma cells in the bone marrow (BM) decreases during therapy but are rapidly restored after treatment. Although the Ab producing cells are regenerated, specific Ab titres remain low at follow up in the ALL children.

Memory B-cells are defined in peripheral blood as being CD27+ B-cells. The soluble form of CD27, sCD27, has been described as an immune activation marker in many different diseases such as HIV and rheumatoid arthritis. The expression of the membrane bound CD27 and its natural ligand CD70 on leukaemic cells was significantly increased as compared to healthy controls and correlated to the levels of sCD27 in serum. In ALL children with the translocation, t(12;21), in the leukaemic pre B-cell clone, sCD27 was significantly higher compared to other leukaemic subtypes. Blocking CD27-CD70 interaction on the leukaemic cells reduces proliferation, which indicates that expression of CD27 and CD70 on these cells is beneficial for the leukaemia.

The survival of memory B-cells and plasma-cells is mediated by different growth factors, cytokines and cell-to-cell contact through different receptors. Many of these factors are produced from stromal cells and other immune cells in the microenvironment. Nerve growth factor (NGF) regulates B cell activation and differentiation and is an autocrine survival factor for memory B-cells. Since memory B-cells are prone to apoptosis during HIV-1 infection, the plasma NGF level was studied and found to be lower in HIV-1 subjects compared to controls.

The addition of recombinant NGF to cultures of purified B cells reduced cell death of memory B cells from HIV-1-infected subjects. Stromal cellderived factor-1 (SDF-1/CXCL12), a chemokine produced by stromal cells, plays an important role in normal B-cell lymphopoesis, migration and burning of preB cells and plasma cells to the BM. At diagnosis of pre-B ALL, serum level of SDF-1 OBS I is elevated in children with ALL compared to healthy children.

SDF-1 OBS ImRNA is present in leukaemic cells derived from children with ALL and might be the source of the elevated SDF-1. In addition, recombinant SDF-1 OBS I enhances pre-B leukaemic cell proliferation in vitro. Cell-to-cell contact between the stromal cells and leukaemic cells regulates the secretion of SDF-1 in our in vitro model indicating the importance of the microenvironment for the pathogenesis of childhood leukaemia.