The Accessory Molecule Lgp55 Plays a Role Early in Murine Fetal Thymocyte Differentiation
The Accessory Molecule Lgp55 Plays a Role Early in Murine Fetal Thymocyte Differentiation
MARCIA MCDUFFIE
WILLIAM T. GOLDE
A rat IgM monoclonal antibody, PA3-795, inhibits the antigen-specific responses of mouse T-cell hybridomas. It recognizes a heavily glycosylated cell-surface protein, designated Lgp55, that is detectable after activation on mature T cells. During fetal life, Lgp55 is found at high levels on newly immigrant thymic T-cell precursors prior to surface expression of other T-lineage molecules. High levels of expression are also found on thymocytes in the outer cortex of adult mice. Thymocytes at later stages of differentiation bear decreasing amounts of surface Lgp55, and none is detectable on "single-positive" thymocytes in the thymic medulla or on resting mature T cells from the periphery. Addition of monoclonal anti-Lgp55 to fetal thymus organ culture decreases the output of "mature" CD4 singlepositive thymocytes when it is begun before fetal day 13.5. These findings suggest that Lgp55 contributes to cell-cell interactions that regulate very early steps in T-cell development in the mouse.
T cells; differentiation; accessory molecules; thymus
INTRODUCTION
Undetectable on resting peripheral mouse T cells,
Lgp55 is expressed at high levels on activated T cells
(both CD4 and CD8) and most T-cell hybridomas.
Binding by monoclonal anti-Lgp55 (PA3-795)
inhibits low-affinity responses of T-cell hybridomas to
antigen presented by B-cell lymphomas (Glode
et al., 1990). Anti-Lgp55 also inhibits proliferation
and the generation of cytotoxic lymphocytes (CTL)
in primary mixed-lymphocyte reactions in vitro
(Golde et al., 1990; M. McDuffie and R. Gill,
unpublished data)
. Thus, the functional evidence to date
indicates a role in the immune response similar to
that of the adhesion molecule LFA-1 and its ligands
(Davignon et al., 1981; Boyd et al., 1988)
. Like
mature T cells after activation, most adult thymocytes
(> 50%) express Lgp55 at moderate to high levels
(Golde et al., 1990)
. This finding raised the
possibility that Lgp55 might enhance interactions between
T-cell precursors and the thymic stromal cells that
direct the developmental sequence.
"Corresponding author.
During murine fetal development, lymphoid
precursors from yolk sac and fetal liver begin to
colonize the murine epithelial thymus structure on
days 11 and 12 of the 19- to 21-day gestation. On
day 12, the thymus is well-colonized with relatively
quiescent lymphoid precursors. By day 14 of
gestation, these lymphoid precursors are actively
proliferating, and the T-cell marker Thy-1 has appeared
on a minority of thymocytes
(Fig. 2; Ceredig et al.,
1983)
. In addition, the earliest rearrangements of
T-cell receptor genes have occurred
(Born et al.,
1986)
and IL-2 message is detectable
(Pardoll et al.,
1987)
. By day 15, mature T cells of the ,c type have
developed, Thy-1 is expressed on the majority of
lymphoid cells, and the differentiation of the first c,8
T cells, which will appear on day 18, is well
underway. The signals that initiate T-cell
development on day 13 have not been determined, but
these steps do not occur to any significant degree in
the absence of a functional thymic epithelium
(Maryanski et al., 1981)
.
To determine the developmental stage in which
Lgp55 might participate, thymocytes from fetal mice
were first examined for expression of Lgp55 in
relation to other markers of thymocyte
differentiation. Because LFA-1 plays a role similar to Lgp55 on
mature T cells and is also expressed on the majority
of thymocytes in the adult mouse
(Golde et al.,
1990)
, its expression was examined along with that
of Lgp55. Detection of high levels of Lgp55 on early
thymic presursors, in advance of other T-cell
markers, prompted and exploration of the possible
role of this accessory molecule in T-cell
differentiation.
RESULTS
Both LFA-1 and Lgp55 were coexpressed on most
thymocytes from fetal mice at day 15 of gestation
(data not shown). However, the thymocyte
precursors found at day 13 expressed only Lgp55 (Fig. 1,
panel A). LFA-1 expression first appears on a
significant proportion of Lgp55 / thymocytes between
days 13 and 14 of fetal development. During the
transition from day 14 through day 16, increasing
percentages of thymocytes coexpress Lgp55 and
LFA-1 (Fig. 1, panels B-D). This period coincides
with rearrangement of all T-cell receptor genes,
ending with rearrangement of the o-chain gene and
the first appearance of functional o,8 T-cell receptor
molecules on the surface of the developing
thymocytes. From the end of day 16 to adulthood, an
increasing percentage of the thymocytes displays
the LFA-lhigh/Lgp55lw-mderate phenotype,
reaching 85% in the adult (Fig. 1, panel E). This
subpopulation of thymocytes must be largely coincident
with, if not identical to, the bulk of nonmature
CD4+/CD8 / "double-positive" cells that also
comprise 85% of the cells in the adult murine thymus
and serves as an intermediate ste (...truncated)