By
From the Department of Immunology, DNAX Research Institute of Molecular and Cellular Biology, Palo Alto, California 94304
Since the initial descriptions of CD4+ T cell subsets with
distinct functions and cytokine production profiles, the
question of how these cells originate during an immune
response has been asked. Both Th1 and Th2 cells can develop from naive, peripheral CD4+ T cell (Thp) populations, and individual Thp cells appear capable of differentiating into either Th subset (reviewed in reference 1). The
differentiation process is initiated by ligation of the TCR,
but additional signals are required for maturation into cells
capable of producing high levels of cytokines upon restimulation. The most clearly defined differentiation inducers
are themselves cytokines: IFN- The principal early events that lead to Th1 differentiation in mice infected with any of a number of intracellular
pathogens are reasonably well understood. These begin
with the production of IL-12 by macrophages responding
either to microbial products or to direct infection and the
subsequent induction, by IL-12, of IFN- The sources and modes of regulation of the initial IL-4
needed to induce Th2 development have, in contrast, not
been so clearly defined. Candidate sources of IL-4 include
atypical subsets of T cells not restricted by class II MHC
(including NK1+ CD4+ T cells), conventional CD4+ memory T cells, eosinophils, and cells of the mast cell/basophil lineage. In this issue, a paper by Rincón et al. (3) now suggests that naive T cells themselves can be the source of IL-4
that leads to their own Th2 development and that IL-6 is a
potent inducer of this IL-4. The basic experimental system
used in this work consisted of purified mouse CD4+ T cells
stimulated in vitro for 4 d. The cells were then harvested, washed, and restimulated, and the supernatants were assayed for IL-4 and IFN- IL-6 is a prominent component of inflammatory and acutephase responses but has not previously been implicated in the
preferential development of either Th2 or Th1 responses (reviewed in reference 4). Rincón et al. (3) suggest the interesting possibility that IL-6 is a key component in a link between
innate immunity and Th2 responses, which parallels the connection between the macrophage/NK response to intracellular pathogens and Th1 induction. Not yet clear, however, are
the specific conditions or pathogens that would favor Th2
over Th1 induction by this pathway. Furthermore, the relatively few published experiments with IL-6 Several other recent studies have addressed the question
of which cell population provides the early source of IL-4
in a Th2 response, and it is becoming evident that there are
multiple answers to the question. The key cell involved in
Th2 induction may depend on the route of immunization or
infection or on the nature and concentration of the antigen.
The cells that have drawn the most attention are NK1+
T cells (7, 8), an unusual T cell population that is mostly
CD4+, expresses several markers characteristic of NK cells,
has a highly restricted TCR V The nonprotective Th2 response to Leishmania major in
BALB/c mice was quite normal in mice rendered deficient
in NK1+ T cells by backcrossing a mutated Evidence remains that conventional CD4+ T cells (meaning CD4+ T cells that are class II MHC-restricted and lack
NK markers) can act as the sources of the initial IL-4 for
Th2 induction. Transient, partial depletion of CD4+ T cells
from BALB/c mice with anti-CD4 antibodies at the time of L. major infection changed these mice from Th2 to Th1
responders (19). This experiment could be interpreted as
showing that the early IL-4 source in this model is a CD4+
T cell, an interpretation supported by direct demonstrations that the basal IL-4 level and the earliest IL-4 induced
by L. major could be abrogated by anti-CD4 treatment (12,
20). Likewise, Th2 development of naive CD4+ T cells
(CD62L+) stimulated in vitro with immobilized anti-CD3
can be driven by IL-4 originating from CD4+ T cells that
had been previously activated, based on their low levels of
CD62L expression (21). In addition to the current paper
from Rincón et al. (3), a few other studies have suggested the possibility that the early source of IL-4 may originate
from naive-responding CD4+ T cell, possibly during a transient multipotential precursor stage of CD4+ T cell differentiation (22, 23).
A different view of the initiation of a Th2 response, at
least for the BALB/c response to L. major, comes from study
of the strong response to the leishmania antigen LACK (24).
L. major infected BALB/c mice make a strong early (6 d)
Th2 response to LACK, although the response of the same
mice to several other L. major antigens is predominantly
Th1-like (25). A role for LACK-specific T cells in the development of a dominant Th2 response to infection is suggested by observation that BALB/c mice expressing a
LACK transgene, and therefore unresponsive to LACK,
develop a dominant Th1 response to L. major infection.
Several interesting questions about this early response to
LACK remain. Does this response originate entirely from
naive Th cells, or do LACK-reactive Th2 cells preexist,
possibly primed by cross-reactive antigens from other environmental antigens? If the LACK-specific Th2 response comes from naive Th cells, does LACK induce unusually
strong or rapid IL-4 production, perhaps in a manner similar to that suggested by Rincón et al.?
Mast cells, basophils, and eosinophils can produce significant amounts of cytokines, including IL-4, upon activation
(26, 27) and can express MHC class II molecules and
present peptides to CD4+ T cells (28, 29). The ability of
mast cells and eosinophils to produce IL-4 and present Ag,
combined with their presence in the peritoneal cavity and at
mucosal sites, makes them candidates to play a role in initiating Th2 differentiation, especially to challenges at these sites.
A recent paper by Sabin et al. (30) provides perhaps the clearest evidence to date of the role of a non-T cell source of
IL-4 in the initiation of a Th2 response. Studying the initial
response to intraperitoneal injection of S. mansoni eggs (a
strong Th2 stimulus), they have shown that the earliest IL-4
originates from eosinophils and that mast cells indirectly
play an important role by secreting IL-5 and recruiting eosinophils to the site of egg injection. Thus, non-T cells
may play a crucial role in the initiation of Th2 responses
against some pathogens in specific tissues.
In conclusion, there appear to be multiple pathways by
which Th2 responses can develop and multiple cellular
sources for the IL-4 responsible for the initial differentiation of Th2 cells from naive precursors. Much remains to
be answered, however, about the specific properties of the
antigens, pathogens, or anatomical sites that are critical to
such Th2 induction.
and IL-12 for Th1, and
IL-4 for Th2 induction. Thus, understanding the cellular
origin and control of production of these cytokines during
a primary immune response is central to understanding the
genesis of Th1 and Th2 responses.
production by
NK cells (reviewed in reference 2). This response usually
occurs within 24 h of infection and constitutes the most effective form of innate immunity for this class of pathogens. Naive T cells initiating a specific response to the pathogen
in a microenvironment dominated by IFN-
and IL-12
develop preferentially into Th1 cells. Thus, the effector
functions of the innate response that are most appropriate
for the control of these pathogens are conserved in the subsequent antigen-specific T cell response.
as measures of Th2 or Th1 differentiation, respectively. Most experiments employed the
polyclonal stimulators Concanavalin A or anti-CD3, but a
few experiments were performed with T cells from a
TCR-transgenic mouse stimulated with the appropriate
peptide epitope. The addition of either IL-6 or IL-4 to the
primary cultures led to a severalfold increase in IL-4 production and a concomitant decrease in IFN-
production
by the cells when restimulated. Significant IL-4 was produced upon restimulation even when no cytokines were added to the primary culture, and this IL-4 was eliminated
either with anti-IL-6 antibodies or by using APC from
IL-6
/
mice. Importantly, the ability of IL-6 to induce
Th2 differentiation was blocked by antibodies to IL-4, implying that IL-6 acted by inducing IL-4 in the primary cultures, which in turn was the direct inducer of Th2 differentiation. In contrast, IL-4 was fully active on CD4+ T cells
from IL-6
/
mice, demonstrating that Th2 induction by
IL-4 did not require IL-6. The evidence that IL-6 induces
IL-4 production directly by the responding naive Th cells
derives from experiments showing the induction of Th2
development in purified naive CD4+ T cells by IL-6.
These results suggest that IL-6 can rapidly induce sufficient
IL-4 from these cells to lead to stable Th2 differentiation,
although this was not directly demonstrated. It should also
be noted that the cultures of naive T cells included whole
splenocytes as the APC, and it was not ruled out that these were the source of IL-4.
/
mice do not
give clear evidence of a defect in Th2 responses (5, 6).
and V
usage, and is restricted by nonclassical class I MHC molecules, including
CD1 (9) and TL (10). The most striking feature of NK1+
T cells, in terms of potential function, is that they can very rapidly produce IL-4 upon activation in vivo with antiCD3 antibodies (11). Thus, NK1+ T cells have the potential to provide IL-4 at the onset of Th2 responses. A number of recent reports, however, fail to support a role for
these cells in the induction of Th2 responses to a variety of
pathogens and protein antigens.
2-microglobulin (
2m) gene onto the BALB/c background (12, 13).
Lack of a requirement for NK1+ T cells in the BALB/c
L. major response was confirmed independently by backcrossing the NK1.1 allele onto the BALB/c background
and depleting mice with an anti-NK1.1 monoclonal antibody (12). Consistent with the lack of effect of these depletions on Th2 development, neither the earliest IL-4 response to infection nor the basal IL-4 mRNA level in
lymph nodes was reduced by the elimination of NK1+ T cells
(12). Responses to the helminth parasites Schistosoma mansoni and Nippostrongylus brasiliensis (13) as well as to protein
antigens continuously delivered by osmotic pumps (14) or
injected subcutaneously (13) or intraperitoneally (15) with
adjuvant were similarly not impaired in
2m
/
mice as
compared with their wild-type controls. It appears, therefore, that IL-4 originating from NK1+ T cells is not required for
the initiation of Th2 responses against antigens administered
locally or against localized chronic infection. Indeed, only the
IL-4 response or the IL-4-dependent IgE response to polyclonal stimuli such as systemic anti-CD3 and anti-IgD have been shown to be dependent on NK1+ T cells (16).
Address correspondence to R.L. Coffman, Department of Immunology, DNAX Research Institute of Molecular and Cellular Biology, 901 California Ave., Palo Alto, CA 94304.
Received for publication 5 December 1996
The DNAX Research Institute is supported by the Schering-Plough Corporation.We thank Drs. Amy Beebe, Stephen Hurst, and Lewis Lanier for helpful comments on the manuscript.
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