Soluble suppressor factor produced by pokeweek mitogen-stimulated human peripheral blood leukocytes

Human suppressor T cells: induction, differentiation, and regulatory functions

T-cell-mediated suppression of human immune responses involves a complex interaction between distinct lymphocyte subsets with suppressor-inducer and suppressor-effector functions. Recent studies with subset-specific monoclonal antibodies have defined a characteristic phenotype of suppressor-inducer cells (CD4+ Leu8+ 2H4+ 4B4-) that can be distinguished from that of helper cells for antibody synthesis (CD4+ Leu8- 2H4- 4B4+). Similarly, suppressor-effector cells (CD8+CD11+Tp44-) can typically be defined as a subset separable from cytotoxic T cells (CD8+CD11-Tp44+). Both antigen-specific and nonspecific interactions are important in suppressor T-cell activation and function. Soluble signals required for differentiation of CD8+ suppressor cells include an indomethacin-sensitive monocyte product and interferon gamma. In contrast, proliferation of the CD8+ suppressor cell subset depends on stimulation first by a product of CD4+Leu8+ cells, T suppressor cell growth factor, and second by interleukin 2. Although the molecular basis of antigen-specific interactions between CD4+ and CD8+ cells in suppressor cell generation has not been defined, it may involve both conventional, presumably MHC-restricted, interactions between antigen and antigen receptors, as well as anti-idiotypic interactions of suppressor-effectors with determinants on suppressor-inducer receptors. Progress in elucidating requirements for activation, growth, and differentiation of suppressor cells should facilitate long-term culture of such cells and lead to clearer understanding of mechanisms of suppressor-cell mediated immunoregulation.

Suppression of polyclonal immunoglobulin production by a soluble factor produced by a human thymus hybridoma

A human thymus cell hybridoma was established using thymus cells obtained from a patient with common variable hypogammaglobulinemia and associated thymoma. This hybridoma secreted a suppressor factor for polyclonal antibody synthesis. Supernatants of this hybrid showed 40-80% suppression of both IgM and IgG synthesis by pokeweed mitogen-stimulated human peripheral blood lymphocytes. Hybridoma supernatants were suppressive for immunoglobulin production only if added within the initial 48 h of the seven-day culture period. Suppression of antibody production by the hybridoma supernatant was prevented by preabsorption with T lymphocytes. Further, the suppressor factor was shown to inhibit antibody production in reconstructed cultures containing T4+ cells and B cells, yet the suppression could be abrogated by increasing the number of T4+ cells. The hybrid supernatant had no affect on the proliferation of human mononuclear cells in response to pokeweed mitogen, lipopolysaccharide, concanavalin A or alloantigen but inhibited phytohemagglutinin-induced proliferation. The target cell population for the inhibition of phytohemagglutinin responsiveness was shown to be a T4+ lymphocyte (helper inducer T cell). These results suggest that thymus hybridoma cells can produce immunoregulatory products that act through the modulation of T4+ lymphocyte function. To our knowledge this is the first human thymus cell hybridoma to be reported. Studies on such cell lines may provide important information on immunoregulatory thymic factors.

High and low density PHA- (but not ConA-) activated T cells stimulate the autologous mixed lymphocyte reaction

In this study, PHA- and ConA-activated cells (PAC and CAC) were used as stimulators in mixed lymphocyte reactions (MLR) using autologous (auto) and allogeneic (allo) peripheral mononuclear cells as responders. PAC, but not CAC, were stimulatory in allo- and auto-MLR, and this stimulation was not due to residual PHA. In PAC which have been activated for 96 h, auto-MLR was due to determinants present on low density T-cell blasts, while with PAC which had been stimulated for more than 192 h, the determinants seemed to be associated with high density T cells. Anti-T3 monoclonal antibodies and certain anti-DR suppressed auto- and allo-MLR mediated by PAC when present throughout the entire MLR assays. CAC suppressed PAC-mediated auto-MLR in a dose-dependent fashion. This inhibition was not DR-restricted and was reversed by the addition of exogenous IL-2. Our results indicate that: depending upon the length of activation, both low density and high density PHA-activated T cells exhibited strong stimulatory capacity in auto-MLR; ConA-activated T cells failed to stimulate auto- or allo-MLR and suppressed MLR mediated by PAC; this suppression was due to suppressor cells, not to suppressor factors, and was readily reversed by exogenous IL-2; pretreatment of CAC with anti-TAC did not reverse the inhibition.

The influence of autologous cell interactions on spontaneous and pokeweed mitogen-induced immunoglobulin production

Both helper- and suppressor-T-cell activities are generated in the autologous mixed lymphocyte reaction and in pokeweed mitogen (PWM)-stimulated cultures. The addition of low numbers of irradiated non-T cells enhance while high numbers suppress spontaneous and PWM-stimulated IgG synthesis by autologous cells. Monocytes are the principal inducers of suppression and exert their influence within the first 24 hr of culture. Suppression in association with PWM stimulation is nonspecific in nature, T-cell mediated, partially radiosensitive, and resistant to hydrocortisone. Neither indomethacin nor dibutyryl cyclic AMP reverses monocyte-related suppression. These findings suggest that the outcome of in vitro Ig synthesis assays is critically dependent upon monocyte-T-cell interaction.

Comparison of the cellular cytotoxic activities of colostral lymphocytes and maternal peripheral blood lymphocytes

Colostral lymphocytes (CL) from mothers 2 to 4 days post-partum and autologous maternal peripheral blood lymphocytes (PBL) were investigated for (1) natural killer (NK) and antibody-dependent cellular cytotoxic (ADCC) activities, (2) target binding ability, (3) interferon (IFN)- and interleukin 2 (IL2)-induced augmentation of NK activity, (4) lectin-dependent cellular cytotoxicity (LDCC), and (5) the ability of culture-derived soluble suppressor factor(s) to inhibit the NK activity of normal allogeneic lymphocytes. CL depleted of adherent cells and Percoll-separated NK-enriched subpopulations of CL demonstrated significantly lower NK and ADCC activities compared to autologous PBL. However, the target binding ability of CL was comparable to autologous PBL. Although the residual NK activity of CL was augmented by IFN and IL2, the activity was not enhanced to the same level shown by autologous PBL. CL also demonstrated a significant enhancement of LDCC activity, although the activity was not stimulated to the levels shown by PBL. Culture supernates of CL manifested greater suppression of the NK ability of allogeneic PBL than culture supernates produced by autologous PBL. These results are consistent with a model that suggests differential partitioning of lymphocyte subpopulations between colostrum and peripheral blood.

A mutant human T-cell line producing immunosuppressive factor(s)

6T-CEM-20, a subclone of a 6-thioguanine-resistant mutant derived from the human-T-cell line CEM, secreted into the medium, a high titered immunosuppressive factor specific for T cells. The cell-free supernatant was very potent in suppressing, via a noncytotoxic mechanism, mitogen-activated T-cell proliferation, cytotoxic T-cell functions, and pokeweed mitogen (PWM)-stimulated plaque-forming cells (PFC). Lower dilutions of the supernatant (10(-1)-10(-2] abrogated T-cell functions within 24 hr whereas higher dilutions (10(-3)-10(-7] required a culture period of up to 4 days with lymphocytes to arrest T-cell activities. The suppressive activity was most pronounced when the factor was added in the early part of the culture period. The factor was sensitive to heat treatment and both low and high pH (most stable at physiological pH). Preliminary purification with column chromatography indicates that the active moiety was contained in the high-molecular-weight fraction (MW greater than 200,000). Data from coculture experiments suggested that T lymphocytes, which were exposed for 5-12 hr to the active supernatant or the partially purified material, suppressed allogeneic T-cell proliferation in a dose-related manner. This suppressor factor which we called suppressor-activating factor (SAF) might have activated a suppressor population or induced the production of a suppressor factor which in turn mediated the observed suppression. Both the molecular structure and the detailed mechanism of action are under investigation.

Mitogen-induced stimulation and suppression of erythroid burst promoting activity production by human mononuclear cells

Exposure of human peripheral blood mononuclear cells or highly enriched monocytes to various plant lectins substantially alters their production of erythroid burst promoting activity (BPA). Neither unstimulated, nor mitogen stimulated, enriched T lymphocytes produced demonstrable BPA. Each of the lectins tested resulted in a different pattern of alteration of BPA production by mononuclear cells. Increasing concentrations of phytohaemagglutinin (PHA) caused a progressive increase in BPA production up to a plateau level at concentrations above 0.25-0.5 microliter/ml. Concanavalin A (Con A) at concentrations of 0.05-0.1 micrograms/ml stimulated BPA production, but Con A concentrations greater than 1 microgram/ml never augmented BPA production by mononuclear cells. Pokeweed mitogen inhibited BPA production by mononuclear cells in a concentration-dependent manner. Since PHA and Con A can bind to and stimulate both monocytes/macrophages and T lymphocytes, some production of BPA by stimulated T cells in the presence of monocytes cannot be ruled out. Earlier studies demonstrated that T cells augment monocyte production of BPA. Thus, monocyte-T cell interactions, as well as activation of monocytes and perhaps lymphocytes, play an important role in regulation of BPA production in vitro.

Inhibition of human antigen-induced lymphoblastoid B-cell function by an in vivo-induced suppressor T cell

Lymphoblastoid (LB) B cells which spontaneously produce antitetanus toxoid IgG antibodies (Tet-IgG) in short-term cultures (3 days) appear in the circulation 5-7 days after immunization with tetanus toxoid. Addition of pokeweed mitogen (PWM), normally a stimulator of antibody production, caused instead a reduction in the in vitro synthesis of Tet-IgG by the LB cells. In order for this inhibition of antibody production to occur, T cells had to be present, and the inhibition was proportional to the number of T cells added to the culture, demonstrating the existence of PWM-inducible suppressor cells. The cells mediating the suppression had the OKT8 phenotype and also exhibited the following characteristics: (1) a PWM pretreatment period as little as 14 hr was enough to complete activation; (2) conventional inhibitors of suppressor T cells as hydrocortisone and cyclosporin A only partially reversed its effect; and (3) DNA synthesis was not required. The T-suppressor activity was detectable in the circulation before immunization, increased two- to fourfold by 5-12 days after boosting, and waned after 3 weeks. The mechanism of action of this suppression does not appear to involve conventional cytotoxic T cells as (1) the suppression was mediated across allogeneic barriers and (2) the suppression could not be reversed by inclusion of anti-Leu-2a antibodies in the culture. These results suggest that this suppressor T-cell subset may be important in the normal regulation of activated stages of human B lymphocytes.