B cell-autonomous somatic mutation deficit following bone marrow transplant = [B cel-autonoom gebrek in somatische mutatie na beenmergtransplantatie]

The bone marrow is the major haematopoietic organ and is critically involved in the production of all formed blood elements in postnatal life. The bone marrow contains rapidly dividing cells and therefore is sensitive to DNA damaging agents. In certain types of cancers where a high dose of radiation and chemotherapeutic agents are needed, a bone marrow transplant is necessary to "rescue" the patient from the lethal side effects of radiation and chemotherapy. However, the immune system of transplant recipients must be regenerated from the transplant inoculum, and it is not surprising that many transplant recipients are deficient in generating specific antibody responses to exogenous stimuli. In this thesis experiments are described which are aimed at a further understanding of the nature of the B cell defects leading to the humoral deficiency in bone marrow transplant (BMT) recipients.

The possibility that an abnormal restriction of the potential antibody repertoire in B cells from BMT recipients could contribute to the deficiency seen in BMT patients is explored in Chapter 3. The results showed that the processes involved in generating and selecting the antibody repertoire are largely functional within the first few months following BMT, and the immunodeficiencies common among BMT recipients probably are not due to the failure to utilize appropriate V region genes in generating the pre-immune antibody repertoire. Rearrangements in BMT recipients, however, exhibited much less somatic mutation than did rearrangements obtained from healthy subjects.

In Chapter 2, the correlation between motif-specific hybridizations and nucleotide sequence variation is described as an approach to identify individual V _H genes. This method is highly specific and allows for the detection of somatic mutation in these genes by sequential hybridization. By using this method, a large number of genes can be analyzed without sophisticated instrumentation.

Somatic mutation of rearranged immunoglobulin genes is a powerful diversifier of the antibody repertoire and an important requisite in the formation of memory B cells. The precise mechanism responsible for turning on the mutational process is unknown. To dissect the role of different components of the germinal center in this mechanism, we have used a system in which freshly isolated, activated CD4 ⁺ T cells drive naïve B cell differentiation over a 14 day culture period. In this system, healthy subject naïve B cells accumulate large numbers of V segment mutations, presumably as a result of activation of the somatic mutation mechanism (Chapter 4). This system is used in Chapter 5 to determine whether the lack of somatic mutation seen in BMT recipients is an intrinsic B cell deficit or secondary to a T cell deficit. The results showed that BMT recipient B cells lack the capacity to accumulate somatic mutation in a T cell dependent manner. This appears to be a B cell autonomous deficit because T cells from some patients were able to support accumulation in heterologous healthy subject B cells, however, they were unable to support the accumulation of mutations in autologous naïve cells.