Читать книгу Neonatal Haematology - Barbara J. Bain, Irene Roberts - Страница 15

The different types of haemopoietic progenitor cell present in fetal life are shown in Figs 1.2 and 1.3. The overall scheme of differentiation of HSC is similar in fetal and adult life. However, recent studies have identified fetal‐specific lymphoid progenitors, including early lymphoid progenitors (ELP) and PreProB progenitors that may be important not only to rapidly boost B cell production during the second trimester, but also to act as targets of leukaemic transformation in infant and childhood ALL.^927–29 ELP are found very early in fetal life (from around 6 weeks post‐conception in the fetal liver and from around 11 weeks post‐conception in bone marrow) but are very rare in adult haemopoietic tissues.²⁹ They are defined both by their immunophenotype (CD34⁺CD127⁺CD19⁻CD10⁻) and their ability to generate B, T and NK cells as well as a small number of myeloid cells.^8,29 PreProB progenitors are one of two types of committed B progenitor cell in fetal life; they lack expression of the CD10 molecule and, like ELP, are very rare in adult bone marrow. By contrast, the second type of B progenitor, the ProB progenitor, is CD10⁺ and is the main, or sole, type of B progenitor found in adult bone marrow.²⁹ It is likely that ProB progenitors lie downstream of PreProB in B lymphoid differentiation and, consistent with this, they have been shown to have undergone complete V_H‐D_H‐J_H rearrangement of their immunoglobulin heavy chain (IgH) loci, in contrast to ELP and PreProB progenitors, which show only partial (D_H‐J_H) IgH rearrangement.⁸ The reasons for the existence of two types of B progenitor and a unique ELP cell in fetal life are unknown but it suggests that there are two pathways of fetal B cell production which may have different physiological roles.

Fig. 1.3 Immunophenotypically defined progenitor populations along the B cell differentiation trajectory in the human fetus. The cell surface markers used to define these populations are shown below each cell type. Based on reference 9.