Thymus Ontogeny in Frogs: T-Cell Renewal at Metamorphosis
Thymus Ontogeny in Frogs: T-Cell Renewal at Metamorphosis
LOUISE A. ROLLINS-SMITH
PATRICK J. BLAIR
A. TRAY DAVIS
Metamorphosis in amphibians presents a unique problem for the developing immune system. Because tadpoles are free-living, they need an immune system to protect against potential pathogens. However, at metamorphosis, they acquire a variety of new adultspecific molecules to which the tadpole immune system must become tolerant. We hypothesized that Xenopus laevis tadpoles may avoid potentially destructive antiself responses by largely discarding the larval immune system at metamorphosis and acquiring a new one. By implanting triploid (3N) thymuses into diploid (2N) hosts, we examined the influx and expansion of host T-cell precursors in the donor thymus of normally metamorphosing and metamorphosis-inhibited frogs. We observed that donor thymocytes are replaced by host-derived cells during metamorphosis, but inhibition of metamorphosis does not prevent this exchange of cells. The implanted thymuses export T cells to the spleen. This donor-derived pool of cells declines after metamorphosis in normally developing frogs but is retained to a greater extent if metamorphosis is inhibited. These studies confirm previous observations of a metamorphosis-associated wave of expansion of T cells and demonstrate that it is not dependent on the relatively high concentrations of thyroid hormones required for metamorphosis. Although some larval T cells persist through metamorphosis, others may be destroyed or the larval population is significantly diluted by the expanding adult population.
Thymus ontogeny; Xenopus laevis; thyroid hormones; metamorphosis
INTRODUCTION
The immune system and immune response
pattern of Xenopus laevis tadpoles are distinct from
that of adults
(reviewed in Flajnik et al., 1987; Du
Pasquier et a1.,1989)
. Tadpoles reject slowly or
more frequently become tolerant of major
histocompatibility complex (MHC)-disparate skin
grafts than adults; and they generally do not
reject skin grafts differing by minor
histoincompatibilities whereas adults do
(Chardonnens and
Du Pasquier, 1973; DiMarzo and Cohen, 1982a,
1982b; Obara et al., 1983; Cohen et al., 1985;
Rollins-Smith et al., 1988)
. The tadpole antibody
response to given antigen is generally less
heterogenous and antibodies have lower
affinities than those of adults
(Du Pasquier and
Haimovich, 1976; Du Pasqu.ier et al., 1979; Hsu
and Du Pasquier, 1984)
. Classical class I MHC
*Corresponding author.
antigens are not expressed by tadpole cells until
they near metamorphosis
(Flajnik et al., 1986;
Flajnik and Du Pasquier, 1988)
and class II MHC
antigens are expressed on different lymphocyte
subpopulations in the tadpole and adult
(Du
Pasquier and Flajnik, 1990; Rollins-Smith and Blair,
1990a)
. All of these studies suggest a significant
reorganization of the immune system at
metamorphosis. Indeed there is evidence for a major
loss of lymphocytes from thymus, spleen, and
liver at metamorphosis
(Du Pasquier and Weiss,
1973; Rollins-Smith et al., 1984; Cohen et al.,
1985)
that is exacerbated by thyroxine-driven
precocious metamorphosis
(Rollins-Smith et al.,
1988)
. As has been shown for birds
(Le Douarin
and Jotereau, 1975; Jotereau and Le Douarin,
1982)
and mammals (Jotereau et al., 1987),
thymus ontogeny in amphibians is characterized by
a succesion of waves of T-cell precursors moving
into the thymus where they expand, differentiate,
and leave as mature T cells
(Turpen and Smith,
1989)
. In X. laevis, the third wave of detectable
expansion occurs in association with
metamorphosis
(Turpen and Smith, 1989)
. Little is known
about the intrathymic signals that result in
attraction of T-cell precursors during receptive
periods. We hypothesized that a thyroid
hormone-dependent maturational event in the
thymus might be responsible for the
metamorphosis-associated period of attractiveness. By
implanting triploid (3N) thymuses into
agematched diploid (2N) larval hosts that were
allowed to metamorphose or were prevented
from metamorphosing, we were able to test this
hypothesis. The studies were designed to answer
two questions. Is the influx and expansion of
stem cells in the thymus at metamorphosis a
thyroid hormone-dependent phenomenon? Do
larval donor-derived T lymphocytes that colonize
the spleen persist through metamorphosis? We
observed that donor thymocytes are replaced by
host-derived cells in about 60 days regardless of
whether they undergo metamorphosis. Some
donor-derived larval T cells persist in the spleen
for at least 1 month after metamorphosis, but
their numbers are low. Inhibition of
metamorphosis results in the persistence of a greater
number of these larval T cells.
RESULTS
Renewal of Thymocytes at Metamorphosis in
Normally Developing Hosts
To study the influx and expansion of T-cell
precursors in normally developing hosts, a 3N
thymus was implanted into a number of age- and
stage-matched 2N hosts at days 35-37 (s (...truncated)