Immune reconstitution after allogeneic hematopoietic stem cell transplantation

Abstract

After treatment with allogeneic hernatopoietic stem cell transplantation (HSCT) the patient suffers from a deficient immune system for at least 12 months or longer. During this period, the patient is susceptible to infections and if the state of immune deficiency is prolonged there is an increased risk of relapse of the underlying malignancy as well as development of secondary malignancies.

The aim of this thesis was to study cellular and molecular mechanisms of the immune reconstitution after HSCT and investigate ways of promoting immune recovery. Previous studies from our group have demonstrated an oligoclonality of the B lymphocyte repertoire after HSCT. Studying the Ig heavy chain CDR3 repertoire in naive and memory B lymphocytes with CDR3 fragment size analysis, we could demonstrate a restricted diversity in memory B lymphocytes compared to naive B cells. At three months after HSCT, this restriction of the memory B cell pool is important to the overall restriction of the B lymphocyte repertoire. T cell function is low at this time point since the number of circulating T cells is decreased and treatment with immunosuppressive drugs inhibits T cell activation. However, T cell dysfunction alone does not explain restriction of the B cell repertoire after HSCT, our data also demonstrate an intrinsic B cell dysfunction.

To determine the role of the conditioning regimen in the immune reconstitution after HSCT, we compared B and T cell recovery after HSCT in patients undergoing HSCT with reduced intensity conditioning or myeloablative conditioning. Comparing circulating levels of B, T and NK cells, no difference was detected between the groups. Analyzing the Ig H CDR3 repertoire with CDR3 fragment size analysis, we demonstrate a delayed recovery of the B cell repertoire, especially in patients undergoing HSCT with reduced intensity conditioning. On the other hand, TcRVbeta CDR3 repertoire recovery was faster in patients undergoing HSCT with reduced intensity conditioning than patients treated with myeloablative conditioning regimen. Thus, our data indicate that immune reconstitution is more dependent on other factors than the type of conditioning used before HSCT.

There is a lot to gain if the period of immune deficiency after HSCT could be shortened or mitigated. Neuroendocrine hormones have been suggested as therapy to promote immune reconstitution after HSCT. We demonstrate altered levels of IGF-1R and TRalpha 1 receptor RNA expression in PBMCs after HSCT. The decreased IGF-1R RNA expression coincided with the expansion of T cells and activated T cells have previously been demonstrated to express low levels of IGF-1R. Therefore, we analyzed the expression of IGF-1R RNA in CD4 positive and CD8 positive T cells with real-time PCR technique. To investigate if GVHD was the T cell activating event, we correlated IGF-1R RNA expression with occurrence of GVHD. We demonstrate a decreased expression of IGF-1R RNA in CD4 and CD8 positive T lymphocytes up to twelve months after HSCT, that was not correlated to acute or chronic GVHD.

Further studies are needed to clarify the role of IGF-1R in immune reconstitution after HSCT. In this thesis we have investigated mechanisms underlying the clinical problem of immune deficiency following HSCT, with advanced immunological and molecular biological methods. With this approach we intend to continue collecting knowledge about immune reconstitution in these severely ill patients and hopefully contribute to improved treatment and care.