"Tolerization" of human T-helper cell clones by chronic exposure to alloantigen

Induction of clonal anergy in T-helper (Th) cells may have a role
in regulating immune responses. A model system for studying Th cell
tolerization at the clonal level in vitro could be useful for
investigating the mechanisms involved. Accordingly, alloreactive
helper cells were maintained in culture with interleukin 2 (IL 2)
by intermittent stimulation with specific antigen. Regardless of
the frequency of antigen stimulation, clones of age less than ca.
35 population doublings (PD) were found to undergo antigen-specific
autocrine clonal expansion in the absence of exogenous IL 2. Such
young clones (designated as phase I) could therefore not be
"tolerized" by frequent exposure to antigen. In contrast, most
clones of age greater than ca. 35 PD could be tolerized by frequent
exposure to antigen (designated as phase II clones). Their
autocrine proliferation was then blocked, although they still
recognized antigen specifically as shown by their retained ability
to secrete interferon-gamma (IFN-gamma) and granulocyte-macrophage
colony stimulating factor (GM-CSF). The mechanism of response
failure involved both an inability to upregulate IL 2 receptors in
the absence of exogenous IL 2, as well as an inability to secrete
IL 2. These defects were not overcome by stimulation with mitogens
or calcium ionophore and phorbol esther in place of alloantigen.
T-cell receptor, alpha, beta, and gamma-chain gene rearrangements
remained identical in phase I and phase II clones. Tolerization of
phase II clones could be avoided by increasing the period between
antigen exposures. Despite this, whether or not phase II cells were
capable of autocrine proliferation, they were found to have
acquired the novel function of inducing suppressive activity in
fresh lymphocytes. Suppressor-induction was blocked by the broadly
reactive MHC class II-specific monoclonal antibody (moAb) TU39, but
not by moAb preferentially reacting only with HLA-DR, DQ, or DP.
Sequential immunoprecipitation on T-cell clones showed the presence
of a putative non-DR, DQ, DP, TU39+ molecule on phase II clones.
However, this molecule was also found on phase I clones. The nature
of the TU39-blockable suppressor-inducing determinant present on
phase II but not on (most) phase I clones thus remains to be
clarified. In addition to suppressor-induction activity, phase II
clones also acquired lytic potential as measured in a lectin
approximation system. Cytotoxic (CTX) potential was also not
influenced by the frequency of antigenic stimulation and could be
viewed as a constitutive modulation of clonal function