Observations of residual differentiation potential during lineage commitment

Area of Science:

• Hematology
• Stem Cell Biology
• Immunology

Background:

• A specific subset of multipotent progenitor cells, identified as common lymphoid progenitors (CLPs), are lineage-negative and express c-kit and Sca-1 but lack Thy 1.1 (Thyneg).
• Previous observations indicated that transplantation of Thyneg cells into lethally irradiated mice resulted in anemia unless supported by competitor marrow, suggesting a lack of erythroid potential.

Purpose of the Study:

• To analyze the erythroid potential of Thyneg cells.
• To compare the erythroid potential of Thyneg cells with the Thy-1.1low (Thylow) subset, which contains hematopoietic stem cells.
• To investigate the timing and nature of lineage commitment.

Main Methods:

• Transplantation of Thyneg and Thylow cells into mice with allelic hemoglobin loci.
• Colony-forming unit-cell (CFU-C) assays in methylcellulose with recombinant cytokines.
• In situ benzidine staining to determine erythroid contribution to colonies.

Main Results:

• Donor-derived hemoglobin was detected in recipients of Thyneg cells, despite high mortality from anemia.
• Recipients of Thylow cells exhibited complete donor-derived engraftment.
• While approximately 60% of early (day 4) colonies from Thyneg cells were hemoglobinized, this decreased to less than 5% by day 11.
• In contrast, over 70% of Thylow-derived colonies remained hemoglobinized.
• A transient appearance of myeloid progeny was also noted in colonies derived from c-kitlow Thyneg cells.

Conclusions:

• These findings demonstrate that commitment to the lymphoid lineage occurs at the Thylow-to-Thyneg interface.
• The loss of erythroid and myeloid potential during this transition is a gradual process, not an abrupt one.
• Transient hemoglobinization in Thyneg colonies may be due to GATA-1 downregulation or signals from non-erythrocytic cells, potentially leading to apoptosis.