The Proliferative Potential of Myeloma Plasma Cells Manifest in the SCID-hu Host

The low proliferative activity of myeloma plasma cells prompted the notion that the clonotypic B cells that exist in the blood and bone marrow of all myeloma patients contain the proliferative myeloma cells (stem cell). We have exploited our severe combined immunodeficiency (SCID)-hu host system for primary myeloma to investigate whether myeloma plasma cells are capable of sustained proliferation. Purified CD38(++)CD45(-) plasma cells consistently grew and produced myeloma and its manifestations in SCID-hu hosts (8 of 9 experiments). In contrast, the plasma cell-depleted bone marrow cells from 6 patients did not grow or produce myeloma in SCID-hu hosts. Similarly, whereas plasma-cell containing blood cells from 4 patients grew and produced myeloma in hosts, neither the PC-depleted blood cells from 3 of the patients nor a blood specimen that did not contain plasma cells grew in SCID-hu hosts, regardless of their CD19-expressing cell contents. Also, in hosts injected with blood cells, although the myeloma cells were able to disseminate through the murine host system, they were only able to grow in the human bones within a human microenvironment and were not detectable in the murine blood or other organs. Interestingly, the circulating plasma cells appear to grow more avidly in the SCID-hu hosts than their bone marrow counterparts, suggesting that they represent a subpopulation of the plasma cells in the bone marrow. Although our studies clearly demonstrate the proliferative potential of myeloma plasma cells, they are suggestive, not conclusive, as to the existence of a preplasmacytic myeloma progenitor cell.