HIV-mediated Expression of Btk in Hematopoietic Stem Cells is not Sufficient to Restore B Cell Function in X-linked Immunodeficient Mice

Correction of B-cell development in Btk-deficient mice using lentiviral vectors with codon-optimized human BTK

X-linked agammaglobulinemia (XLA) is the most common primary immunodeficiency (PID) in man and caused by mutations in the Bruton's tyrosine kinase (BTK) gene. XLA is characterized by a B-cell differentiation arrest in bone marrow, absence of mature B cells and immunoglobulins (Igs), and recurrent bacterial infections. We used self-inactivating lentiviral vectors expressing codon-optimized human BTK under the control of three different ubiquitous or B cell-specific promoters. Btk-/- mice engrafted with transduced cells showed correction of both precursor B-cell and peripheral B-cell development. Lentiviral vectors containing the wildtype BTK sequence did not correct the phenotype. All treated mice with codon-optimized BTK exhibited the recovery of B1 cells in the peritoneal cavity, and of serum IgM and IgG3 levels. Calcium mobilization responses upon B-cell receptor stimulation as well as in vivo responses to T cell-independent antigens were restored. Viral promoters overexpressing BTK >100-fold above normal resulted in erythro-myeloid proliferations independent of insertional mutagenesis. However, transplantation into secondary Btk-/- recipients using cellular promoters resulted in functional restoration of peripheral B cells and IgM levels, without any adverse effects. In conclusion, transduction of human BTK corrects B-cell development and antigen-specific antibody responses in Btk-/- mice, thus indicating the feasibility of lentiviral gene therapy for XLA, provided that BTK expression does not vastly exceed normal levels.

Hematopoietic engraftment of XLA bone marrow CD34(+) cells in NOG/SCID mice

BACKGROUND AIMS

X-linked agammaglobulinemia (XLA) is a rare primary immunodeficiency associated with mutations of the BTK (Bruton agammaglobulinemia tyrosine kinase) gene. Non-functional BTK leads to a severe decline in peripheral B cells and profound pan-hypogammaglobulinemia. Substitutive immunoglobulin replacement therapy improves long-term survival but remains a symptomatic rather than curative treatment that does not provide an optimal quality of life. Hematopoietic stem cell gene therapy represents a potentially curative treatment. Thorough pre-clinical testing of innovative therapies requires that adequate disease models are available. Invalidation of the murine btk gene produces a phenotype that is less severe than the human disease; alternatively, xenotransplantation of human hematopoietic progenitors obtained from XLA patients may provide a model for testing new treatment procedures.

METHODS

The standard of care for XLA patients rarely offers an opportunity to collect peripheral blood or bone marrow (BM) hematopoietic progenitors; however, we had access to two BM samples obtained from such individuals. We analyzed hematopoietic engraftment of immunoselected CD34(+) cells from these samples in NOD/SCID/ gammac(null) (NOG) mice.

RESULTS

In both cases, human hematopoietic cells were readily detected in BM and thymus, and at low levels in spleen and peripheral blood. Unexpectedly, the early defect in B-lymphoid differentiation associated with XLA was not accurately reproduced in NOG mice, as large amounts of pre-B cells were found in BM.

CONCLUSIONS

These results support the existence of differences in environmental regulation of B-cell ontogeny between mice and humans. This questions the relevance of the NOG xenograft model for pre-clinical study of XLA gene therapy.