Cellular control of tolerance maintenance

Active immunological suppression was first established in 1946 by Chase using guinea pigs fed contact sensitising agents. Not until 1976 was it determined that T cells were responsible for this suppression, using adoptive transfer studies of splenocytes from bovine serum albumin (BSA) fed rats (Thomas and Ryan 1976). It was also observed that this tolerance could not be abolished on administration of naive spleen cells (Richman et al 1978). Subsequent research attempted to establish the phenotype of these cells. It was repeatedly shown that this adoptive transfer of suppression could be abolished by prior treatment of tolerised splenocytes in vitro

with anti Thy-1.2 antibody and complement (Ngan and Kind 1978, Richman et al

1978, M iller and Hanson 1979, Mowat 1985 and Kay and Ferguson 1989). Thymectomy was also shown to comprom ise the induction o f oral tolerance (Gautam and Battisto 1985), whereas removal of B cells from splenocytes obtained

from tolerant donors by prior passage over nylon wool had no effect on subsequent transfer of tolerance (Richman et al 1978). This verified the hypothesis that T cells were important in oral tolerance induction and maintenance. However, adoptive transfer of cells appeared only to transfer tolerance of cellular immunity when soluble protein antigens were used as tolerogens (Mowat 1985), a finding which corresponds with serum transfer of oral tolerance which also only transfers DTH tolerance into naive recipients (Strobel et at 1983).

It has been established that this tolerant state is acutely radiosensitive and tolerance is not maintained in irradiated animals when reconstituted with naive cells (Hanson

et al 1979a). Similarly treatment with cyclophosphamide (CY) (Mowat and Ferguson 1981, Mowat et al 1982, Mowat and Ferguson 1982, Strobel et al 1983) and 2'deoxyguanosine (Mowat 1986a), proposed inhibitors of T cells with suppressor activity, have been shown to abolish oral tolerance of DTH but have very little effect on antibody tolerance. However, these results require careful interpretation since the gastrointestinal tract is very susceptible to damage by radiation and chemicals due to its high cell turnover (Mowat 1986a).

In 1982 Green et al demonstrated that complement lysis of CD8 positive cells in vitro abrogated transfer of oral tolerance, an effect which was attributed to the presence of classical T suppressor cells. Subsequent research attempted to establish an active suppressor cell network similar to that reported in other peripheral toler­ ance models. Using sRBC as an antigen it was suggested that the presence of T contrasuppressor cells in the mucosal microenvironment permitted induction of the local immune response while Peyers patch T suppressor cells maintained systemic unresponsiveness (Gershon et al 1981, Green et al 1982, Taylor 1986). The apparent absence of T suppressor cells in the Peyers patch of mutant LPS unresponsive mice was proposed as an explanation of their failure to become orally tolerised using sRBC as the antigen (Wannemeulher et al 1986). However oral tolerance induced using soluble protein antigens e.g. OVA fed to LPS unresponsive mice produced systemic tolerance of both the humoral and cellular immune responses (Mowat

1986c).

On the basis of this conflicting data it appears that particulate antigens such as sRBC are perhaps not suitable antigens for demonstrating typical oral tolerance to common food antigens. The mechanisms which maintain tolerance towards normal gut flora and multi-antigenic particulate antigens (e.g. sRBC) may be very different to the processes which maintain systemic tolerance to soluble protein antigens.

Hanson et al (1979b) demonstrated that splenectomy before the oral administration of antigen had no effect on the level of tolerance subsequently induced. However, splenocytes transfer specific tolerance of cellular immunity very efficiently (Parks and Weigle 1980, M iller and Hanson 1979, Richman et al 1978). These two observations can be explained by the migration of tolerising cells from the mucosal immune tissues to the spleen after oral administration of antigen. If this is the case it suggests that the induction of T suppressor cells occurs in the local environment of the gut and then migrate to the periphery. This hypothesis is confirmed by the observation that transfer of tolerance in mice by spleen cells is dependent on the time of transfer after feeding the donor animal. Spleen cells taken earlier than day 7 after feeding are unable to transfer tolerance although mesenteric lymph node cells can transfer tolerance only 3 days after a feed (Miller and Hanson 1979). From this data it would appear that these T suppressor cells are induced in the mucosal immune system and subsequently migrate to the periphery via the mesenteric lymph nodes into the spleen.

Although it has been repeatedly stated that T cells are a prerequisite for the induction of oral tolerance, other immune cells and their status may also play a contributing role in the level of tolerance induced and the length of time that tolerance is main­ tained.

Treatment of mice with oestradiol abrogated oral tolerance induction with no demonstatable effect on T cells (Mowat and Parrott 1983). This was thought to be due to the activation of specialised antigen processing cells able to present an immunogenic epitope of the antigen rather than a tolerogenic epitope to T cells. This was also the suggested explanation for the observation that during a mild graft versus

host reaction (GvHR) oral tolerance to ovalbumin could not be induced (Strobel et al

1985). However, as previously mentioned, it is necessary to re-evaluate the effect of these treatments on the activation of APC and their antigen presentation efficiency in context with the changes of the mucosal architecture and integrity, after treatment with oestradiol and during GvHR (Strobel et at 1985).

An experimental split in tolerance induction is observed in many different models of oral tolerance with induction of DTH tolerance but no tolerance of the specific antibody response (Strobel et al 1983, Mowat 1985). This observation may parallel the role of subpopulations of the murine T helper cell population. Two major forms of T helper cell have been demonstrated in mice, primarily by their differential lymphokine production (Mossman and Coffman 1987). On antigenic stimulation T helper 1 (Thl) cells produce rIFN and IL-2, the principle cytokine mediators of DTH, whereas the T helper 2 (Th2) subset produce IL-4 one of the major mediators of T cell help for B cells responding to T-dependent antigens. These T helper subsets have been the focus of intensive research into T cell mediated phenomena but their role in oral tolerance has not been investigated. Other in vitro models of tolerance have established that both Thl and Th2 subsets can be tolerised but they exhibit distinct differences in their requirements for antigen and APC for tolerance induction (O’Hehir et al 1991).