Monoclonal IgG kappa gammopathy previous to hematopoietic stem cell transplantation in an infant with severe combined immunodeficiency.

Severe combined immunodeficiency (SCID) comprises a group of genetic disorders characterized by impaired cellular and humoral immunity due to a block in T lymphocyte differentiation, variably associated to abnormal development of other lymphocyte lineage. The estimated incidence is 1 in 75,000– 100,000 births. It is characterized by early onset infections that can be caused by low-virulence pathogens [1]. Spontaneous engraftment of maternal T‐cells through transplacental transfer occurs in approximately 50% of the cases, and is mostly asymptomatic [1,2]. The most common curative therapy for all forms of SCID is hematopoietic stem cell transplantation (HSCT) [3]. Without immune reconstitution, SCID is inexorably fatal, mostly within the first year of life [4]. Benign monoclonal or polyclonal gammopathies and lymphoprolipherative diseases have been described in malignancies, auto-immune diseases, aplastic anemia and after HSCT in patients with primary or secondary immunodeficiencies. It is thought that they are secondary to monoclonal B‐cell proliferation, as a consequence of an abnormal regulatory T‐cell function [5–7]. In SCID patients, there is only one case report of monoclonal IgA gammopathy prior to HSCT in a T-B‐SCID with maternal B-lymphocyte engraftment, and uncontrolled proliferation [4]. To our knowledge, this is the first published case of an infant with T-B+NK‐SCID (X-linked) who developed a monoclonal IgGK gammopathy derived from self B‐cells prior to HCT that completely resolved after HCT when full engraftment of donor T-cells occurred.