Page 1178 - Williams Hematology ( PDFDrive )

1152 Part IX: Lymphocytes and Plasma Cells Chapter 74: Lymphopoiesis 1153

lymphoid commitment, was held long before cells that satisfied the heavily to lymphoid cells; after transplantation into irradiated recipients,
criteria of CLP were identified. 45–47 In contrast, the existence of single LSK CD34+FLT3 cells rapidly reconstitute B and T lymphopoiesis.
hi
clonogenic cells with myeloid, erythroid, and megakaryocyte lineages However, unlike the multipotent LSK CD34+FLT3 cells, reconstitu-
neg
hi
was shown more than three decades ago through the use of in vitro tion of myeloid lineages in vivo from LSK CD34+FLT3 is very limited,
clonal assays to demonstrate so-called colony-forming unit-granulocyte and importantly lacks granulocytic potential. Indeed, subsequent in
55
48
erythromyeloid megakaryocyte, and later confirmed using markers to vivo fate-mapping studies have questioned the physiologic importance
50
49
prospectively isolate such cells in mice and humans. The lymphocyte of a lymphomyeloid differentiation pathway to steady-state myeloid
lineages were assumed to be closely related because of a number of asso- development, both in the marrow and thymus. 59,60
ciations, for example, common anatomical sites of T and B lymphopoie- One confusing factor in defining lineage potential is the ability
sis (spleen, lymph nodes; Chap. 6), similar molecular mechanisms that of both myeloid and lymphoid progenitor populations to differentiate
regulate T-cell receptor and B-cell immunoglobulin rearrangements into DCs, at least in vitro (Chap. 21). 61–63 As in vitro-derived DC express
(Chaps. 75 and 76), and severe B- and T-lymphoid defects that result many cell-surface markers common to myeloid antigen-presenting cells,
from single genetic mutations in mice (Chap. 80). 51 irrespective of the lineage of origin, the extent to which identification of
Development of flow cytometry (synonym: fluorescence-activated in vitro myeloid potential is confounded by a DC program is unclear.
cell sorting [FACS]) made possible the isolation of rare hematopoietic
cell populations and the subsequent interrogation of lineage poten-
tial using in vitro cultures and in vivo reconstitution studies (Chap. HUMAN LYMPHOID PROGENITORS
18). Primitive multilymphoid progenitors with little or no clonogenic The CD34 cell-surface marker is expressed on human HSCs and on
myeloid or erythroid potential have now been isolated from human a variety of different types of hematopoietic progenitors, including
tissue using flow cytometry with combinations of various cell-surface those restricted to lymphoid development (Chap. 18). 64,65 CD34 has
markers. 52–54 However, it seems likely that lineage relationships are less- been combined with additional cell-surface markers to identify human
strictly organized than once believed. Studies in mice show that the ery- multilymphoid progenitors, for example, CD10, 52,66 CD7, 53,54,67,68 and
throid and megakaryocytic lineages can branch off at an earlier point in CD45RA. 52,67 Similar to the mouse CLP, in addition to the upregulation
hematopoiesis, and that lymphoid (i.e., T, B, and NK) and myeloid (or of expression of the aforementioned markers, lymphoid commitment is
at least monocytic) lineages can arise from the same pathway through accompanied by downregulation of certain HSC cell-surface markers,
a so-called lymphoid-primed multipotent progenitor (LMPP). It such as c-kit and Thy-1. 41
55
remains unclear which of these lineage differentiation pathways are As in murine studies, no one marker used in isolation is able to
most physiologically significant during steady-state hematopoiesis, but define a human lymphoid progenitor. For example, although the
65
it is likely that more than one pathway can exist simultaneously and expression of CD7 can be used to define a subset of CD34+lin CD38
neg
neg
alternative pathways may predominate during different stages of ontog- cells in cord blood that are multilymphoid progenitors without mye-
eny and from different sites of hematopoiesis. loid or erythroid potential, 53,54 CD34+lin CD38+CD7+ cells from cord
neg
blood have full lineage (lymphoid, myeloid, and erythroid) potential.
MURINE LYMPHOID PROGENITORS Furthermore, when comparing the progenitor populations identified
In 1997, investigators working with murine marrow cells, isolated pro- in human studies with those described from murine experiments it
genitors that possessed no myeloid or erythromegakaryocytic potential, is important to recognize that species differences exist between cell-
but when transplanted into irradiated recipients could rapidly restore surface markers. For example, IL-7Rα expression is used to define
41
T-, B-, and NK cell lineages. Clonal in vitro and in vivo studies showed murine CLP, but CD34+lin CD38 CD7+ multilymphoid progeni-
neg
neg
56
56
that all lymphoid lineages were derived from a single common pro- tors in human cord blood do not express IL-7Rα, and CD34+lin C-
53
neg
genitor, thus proving the existence of the long-assumed CLP and sup- D38+IL-7Rα+ cells in human cord blood have both myeloid, lymphoid
porting the classical model of lymphopoiesis. This study isolated cells and even some erythroid potential. A different ontogeny and source
based in part on expression of interleukin-7 receptor alpha (IL-7Rα). of hematopoietic cells will also introduce unexpected variations of
56
The IL-7Rα+ CLP do not express hematopoietic markers associated progenitor immunophenotype and function. Whereas most murine
41
with fully differentiated lineages (they are called “lineage negative” or studies have been conducted with adult murine marrow, most human
“lin ” cells). As an indication that they are more differentiated than studies have been performed with umbilical cord blood, a more logis-
neg
multilineage HSCs, expression of certain HSC-related cell-surface tically available source containing progenitors that are significantly
markers (Sca-1, Thy-1, c-kit) is downregulated. Thus the full immuno- more proliferative than marrow. Again using the example of the
69
56
phenotype assigned to the murine CLP is Lin IL-7Rα+ Thy-1 Sca-1 CD34+lin CD38 CD7+ multilymphoid progenitor, although this
lo
neg
neg
neg
neg
c-kit . This contrasts the murine CLP immunophenotype with that of immunophenotype can be used to identify multilymphoid progenitors
lo
the murine HSC, which is found within the Lin IL-7Rα Thy-1 Sca- in cord blood, the same markers cannot be used in human marrow
53
neg
neg
lo
1 c-kit population. 56 because CD34+lin CD38 marrow cells do not express CD7. Two
hi
neg
hi
neg
Work with murine marrow has prompted a reexamination of when candidates for the human equivalent of the murine LMPP have been
the lymphoid lineage pathways diverge from those of the myeloid and described, both of which coexpress FLT3: one in the marrow identified
erythroid lineages (see Fig. 74–2). A population from murine mar- as CD34+lin CD38+CD45RA+ and high expression of CD62L (L-
neg
row cells, defined largely by expression of the receptor FLT3 (Fms-like selectin) ; and another in the marrow and cord blood characterized as
70
tyrosine kinase 3), has been shown to possess full lymphoid and some a CD34+lin CD38 Thy-1 lo/neg CD45RA+ multilymphoid progenitor
neg
neg
myeloid potential, but not erythroid or megakaryocytic potential. 55,57 (MLP). The hierarchical relationship of these two populations to each
71
These lin sca-1+c-kit+CD34+FLT3 (synonym: LSK CD34+FLT3 ) other, or to the marrow CD34+lin CD38+CD45RA+CD10+ CLP is
hi
neg
neg
hi
cells are primed for lymphoid commitment in that they have down- unclear; and as in the murine system, the physiologic contributions of
regulated genes involved in erythromegakaryocytic differentiation and a lymphomyeloid developmental pathway to steady-state hematopoiesis
upregulated lymphoid-associated genes. They have thus been termed has yet to be determined. Adding to the concept of possible indepen-
58
LMPPs. 55,58 Although they are able to generate monocytes and gran- dence of dendritic cell development, DC potential has been found in all
ulocytes in vitro, their differentiation potential is nonetheless skewed the primitive human lymphoid progenitors reported. 52,53,67,70–72
Kaushansky_chapter 74_p1149-p1158.indd 1153 9/18/15 2:26 PM