Enhancement Of Delayed-type Hypersensitivity Research Articles

Evaluation of the Effect of Hydroethanolic Root Extract and Solvent Fractions of Cyphostemma adenocaule (Steud. ex A. Rich) Descoings ex Wild & Drummond on Cell-Mediated Immune Response and Blood Cell Count in Mice.

Introduction Cyphostemma adenocaule (Steud. ex A. Rich) Descoings ex wild & Drummond (Vitaceae) has been used in traditional medicine for the management of various immunological and hematological disorders in different areas of Ethiopia and the rest of the world. In Ethiopia, the plant is used for the management of enlarged spleen, rabies virus, helminthic infection, snake bite, and various types of tumors. Objective To evaluate the effect of hydroethanolic root extract and solvent fractions of Cyphostemma adenocaule on cell-mediated immunity (delayed-type hypersensitivity), organ index (spleen and liver), and blood cell count in Swiss albino mice. Materials and Methods Acute oral toxicity test was conducted using nulliparous and nonpregnant Swiss albino mice following OECD 425 limit test method. Delayed-type hypersensitivity model was used to evaluate the effect on cell-mediated immunity. The experimental animals were divided into twelve groups which were sensitized and challenged with sheep red blood cells on day 0 and day 7, respectively. Levamisole 50 mg/kg was used as stimulant control, whereas cyclophosphamide 30 mg/kg was used as suppressant control. Hydroethanolic root extract (100 mg/kg, 200 mg/kg, and 400 mg/kg), aqueous fraction (100 mg/kg, 200 mg/kg, and 400 mg/kg), and n-butanol fraction (100 mg/kg, 200 mg/kg, and 400 mg/kg) were administered for seven days. The paw volume was measured using a digital plethysmometer before challenge and 24 hours after challenge. Blood was collected, and organs (spleen and liver) were isolated from each challenged mouse to determine blood cell count and organ index, respectively. Results No mortality and noticeable behavioral changes were observed among all mice receiving hydroethanolic root extract and solvent fractions at a dose of 2000 mg/kg. Hydroethanolic root extract and solvent fractions of Cyphostemma adenocaule showed enhancement of delayed-type hypersensitivity, organ index, and blood cell count. Hydroethanolic root extract at a dose of 400 mg/kg showed the highest and statistically significant stimulation of delayed-type hypersensitivity (0.123 ± 0.010) and blood cell count compared to the vehicle. Conclusion Hydroethanolic root extract and solvent fractions of Cyphostemma adenocaule showed a stimulatory effect on cell-mediated immunity and hematopoiesis.

Lymphokine-activated killer (LAK) cells modulate the effects of IL-2 on a T cell-mediated immune response

The ability of LAK cells and/or IL-2 to affect the course of an established T cell response was examined in a delayed-type hypersensitivity (DTH) model. IL-2 greatly increased the magnitude of the response at 24 h, while LAK cells alone had no effect. The administration of LAK cells and IL-2 together also had no effect on the magnitude of the DTH response, demonstrating that LAK cells were able to remove the enhancement seen with IL-2 alone. The presence of LAK cells reduced the serum half-life of IL-2 significantly, but not to an extent able to account for the observed loss of IL-2 induced DTH enhancement. IL-2 administration influenced cell phenotypes in the spleen and draining lymph nodes (DLN), as well as increasing splenic weight; the additional presence of LAK cells markedly altered these effects of IL-2 in the spleen (but not the DLN). Taken together, these results suggest that LAK cells interact with activated T-cells within the immune system and modulate their function.

The Paradoxical Effects of Lead in Interferon-Gamma Knockout BALB/c Mice

It has been reported that lead (Pb) exposure enhances interleukin (IL)-4 and inhibits interferon-gamma (IFNgamma) production in wild-type (WT) BALB/c mice. Here, we examined Pb effects on immunity in IFNgamma knockout (KO) mice. Lead significantly enhanced serum IgG1 anti-keyhole limpet hemocyanin (KLH) levels in WT mice compared to the controls; Pb also increased serum IgG2a anti-KLH levels, but the IgG1:IgG2a ratio was greater with Pb. In addition, total serum IgE levels, but not IgE anti-KLH levels, were increased. In the KO mice, the serum IgG1, IgG2a, IgE anti-KLH, and total IgE levels were significantly lower than those of WT mice. Surprisingly, Pb significantly enhanced IgG1 and IgG2a anti-KLH levels in the KO mice. However, for these mice, unlike the WT mice, Pb caused a greater percentage change in IgG2a than in IgG1 anti-KLH, indicating less skewing toward type-2 immunoglobulins. Lead also enhanced the delayed-type hypersensitivity (DTH) response in WT mice. Not surprisingly, very low DTH occurred in the KO mice; however, Pb induced a strong KLH-specific DTH response. The in vivo Pb exposure significantly increased in vitro production of IL-4, IL-5, and IL-10, but not IFNgamma, IL-2 and IL-12, by KLH-induced WT and KO spleen cells. In contrast to KLH, dinitrofluorobenzene contact hypersensitivity (DNFB CHS) was detected in all groups, and Pb did not affect this response, which suggests that Pb has only a slight effect on CD8+ T cell-related responses. As previously reported, Pb enhances Th2 responses in WT mice; however, in the KO mice, Pb enhanced Th1-related anti-KLH production and a Th2-related DTH. The Pb enhancement of DTH in IFNgamma-deficient mice is likely due to promotion of type-2 cytokines and enhancement of major histocompatibility complex (MHC) class II expression.

Stress, leukocyte trafficking, and the augmentation of skin immune function.

Delayed type hypersensitivity (DTH) reactions represent cell-mediated immune responses that exert important immunoprotective (resistance to viruses, bacteria, and fungi) or immunopathologic (allergic or autoimmune hypersensitivity) effects. We have used the skin DTH response as an in vivo model to study neuro-endocrine-immune interactions. We hypothesized that just as an acute stress response prepares the cardiovascular and musculoskeletal systems for fight or flight, it may also prepare the immune system for challenges (e.g., wounding) that may be imposed by a stressor (e.g., an aggressor). Studies showed that acute (2 hours) stress experienced before primary or secondary cutaneous antigen exposure induces significantly enhanced skin DTH. This enhancement involves innate as well as adaptive immune mechanisms. Adrenalectomy eliminates the stress-induced enhancement of DTH. Acute administration of physiological concentrations of corticosterone and/or epinephrine to adrenalectomized animals enhances skin DTH. Compared with those in controls, DTH sites from acutely stressed or hormone-injected animals show significantly greater erythema and induration, numbers of infiltrating leukocytes, and levels of cytokine gene expression. In contrast to acute stress, chronic stress is immunosuppressive. Chronic exposure to corticosterone or acute exposure to dexamethasone significantly suppresses skin DTH. These results suggest that during acute stress, endogenous stress hormones enhance skin immunity by increasing leukocyte trafficking and cytokine gene expression at the site of antigen entry. Elucidation of mechanisms mediating a stress-induced enhancement of skin immune function is important because such immunoenhancement can have protective (wound healing, resistance to infection) or pathological (allergic or autoimmune hypersensitivity) consequences.

Stress-induced augmentation of immune function—The role of stress hormones, leukocyte trafficking, and cytokines

Delayed-type hypersensitivity (DTH) reactions represent cell-mediated immune responses that exert important immunoprotective (resistance to viruses, bacteria, and fungi) or immunopathological (allergic or autoimmune hypersensitivity) effects. We initially utilized the skin DTH response as an experimental in vivo model to study neuro–endocrine-immune interactions in rodents. We hypothesized that just as an acute stress response prepares the cardiovascular and musculoskeletal systems for fight or flight, it may also prepare the immune system for challenges which may be imposed by a stressor. The skin DTH model allowed us to examine the effects of stress at the time of primary and secondary exposure to antigen. Studies showed that acute (2 h) stress experienced before primary or secondary antigen exposure induces a significant enhancement of skin DTH. Importantly, this enhancement involved innate as well as adaptive immune mechanisms. Adrenalectomy eliminated the stress-induced enhancement of DTH. Acute administration of physiological (stress) concentrations of corticosterone and/or epinephrine to adrenalectomized animals enhanced skin DTH. Compared with controls, DTH sites from acutely stressed or hormone-injected animals showed significantly greater erythema and induration, numbers of infiltrating leukocytes, and levels of cytokine gene expression. In contrast to acute stress, chronic stress was immunosuppressive. Chronic exposure to corticosterone, or acute exposure to dexamethasone significantly suppressed skin DTH. These results suggest that during acute stress, endogenous stress hormones enhance skin immunity by increasing leukocyte trafficking and cytokine gene expression at the site of antigen entry. While these results are discussed from a mechanistic and clinical relevance perspective, it is acknowledged that much work remains to be done to elucidate the precise mechanisms mediating these bi-directional effects of stress and stress hormones and their clinical ramifications.

A Hassle a Day May Keep the Doctor Away: Stress and the Augmentation of Immune Function

Stress may be defined as a sequence of events, that begins with a stimulus (stressor), that is recognized by the brain (stress perception), and which results in the activation of physiologic fight/flight/fright systems within the body (stress response). Many evolutionary selection pressures are stressors, and one of the primary functions of the brain is to perceive stress, warn the body of danger, and enable an organism to respond. We hypothesized that under acute conditions, just as the stress response prepares the cardiovascular and musculoskeletal systems for fight or flight, it may also prepare the immune system for challenges (e.g., wounding) which may be imposed by a stressor (e.g., an aggressor). Initial studies showed that acute (2h) stress induced a significant trafficking of immune cells to the skin. Since the skin is an organism's major protective barrier, we hypothesized that this leukocyte redistribution may serve to enhance skin immunity during acute stress. We tested this hypothesis using the delayed type hypersensitivity (DTH) reaction, which mediates resistance to various infectious agents, as a model for skin immune function. Acute stress administered immediately before antigen exposure significantly enhanced skin DTH. Adrenalectomy (ADX) eliminated the stress-induced enhancement of DTH while administration of physiological doses of corticosterone and/or epinephrine to ADX animals enhanced skin DTH in the absence of stress. These studies showed that changes in leukocyte distribution and circulating stress hormones are systemic mediators of the immunoenhancing effects of acute stress. We recently identified gamma interferon as a local cytokine mediator of a stress-induced immunoenhancement. Our results suggest that during acute stress the brain sends preparatory warning signals to the immune system just as it does to other fight/flight systems of the body.

Acute stress enhances while chronic stress suppresses skin immunity. The role of stress hormones and leukocyte trafficking.

Delayed-type hypersensitivity (DTH) reactions are antigen-specific, cell-mediated immune responses that, depending on the antigen, mediate beneficial (resistance to viruses, bacteria, fungi) or harmful (allergic dermatitis, autoimmunity) aspects of immunity. Contrary to the widely held notion that stress is immunosuppressive, we have shown that under certain conditions, stress can enhance immune function. DTH reactions can be studied in rats or mice by challenging the pinnae of previously sensitized animals with antigen. Studies have shown that acute stress administered immediately before antigen exposure significantly enhances skin DTH. In contrast, chronic stress significantly suppresses skin DTH. Stress-induced changes in leukocyte distribution may contribute to these bidirectional effects of stress, since acute stress induces a significant mobilization of leukocytes from the blood to the skin, whereas chronic stress suppresses leukocyte mobilization. In order to identify the hormonal mediators of the observed effects of stress, we first showed that adrenalectomy (ADX) eliminates the stress-induced enhancement of DTH. Acute administration (to ADX animals) of low doses of corticosterone and/or epinephrine significantly enhances skin DTH. In contrast, acute administration of high doses of corticosterone, low doses of dexamethasone, or chronic administration of moderate doses of corticosterone, suppress skin DTH. Thus, the timing and duration of stress may significantly affect the nature (enhancing versus suppressive) of the effects of stress on skin immune function. These results suggest that during acute stress, stress hormones may help enhance immune function by informing the immune system about impending challenges (e.g., wounding or infection) that may be imposed by a stressor (e.g., an aggressor). Thus, during acute stress, the brain may send a warning signal to the immune system, just as it does to other fight/flight systems in the body.

Delayed-type hypersensitivity elicited by synthetic peptides complexed with Mycobacterium tuberculosis hsp 70.

Four synthetic peptides bearing dominant CD4+ T-cell epitopes of the 19,000 and 38,000 MW proteins of Mycobacterium tuberculosis were used to provoke a delayed-type hypersensitivity (DTH) reaction in mice previously immunized with recombinant 19,000 and 38,000 MW proteins. It was found that an effective enhancement of the DTH reaction was obtained if the peptides were administered as a complex with M. tuberculosis hsp 70 protein. The increase in reactivity was not obtained when hsp 70 and peptide were co-injected at the same site, but not in complex, or when the specific peptide was displaced from the complex by an irrelevant peptide with high capacity to bind hsp 70. One of the antigenic peptides whose capacity to complex with hsp 70 is low, failed to show the enhancement of DTH when injected together with hsp 70.

S6-3 The aging effects on learning and memory

Delayed-type hypersensitivity (DTH) reactions represent cell-mediated immune responses that exert important immunoprotective (resistance to viruses, bacteria, and fungi) or immunopathological (allergic or autoimmune hypersensitivity) effects. We initially utilized the skin DTH response as an experimental in vivo model to study neuro–endocrine-immune interactions in rodents. We hypothesized that just as an acute stress response prepares the cardiovascular and musculoskeletal systems for fight or flight, it may also prepare the immune system for challenges which may be imposed by a stressor. The skin DTH model allowed us to examine the effects of stress at the time of primary and secondary exposure to antigen. Studies showed that acute (2 h) stress experienced before primary or secondary antigen exposure induces a significant enhancement of skin DTH. Importantly, this enhancement involved innate as well as adaptive immune mechanisms. Adrenalectomy eliminated the stress-induced enhancement of DTH. Acute administration of physiological (stress) concentrations of corticosterone and/or epinephrine to adrenalectomized animals enhanced skin DTH. Compared with controls, DTH sites from acutely stressed or hormone-injected animals showed significantly greater erythema and induration, numbers of infiltrating leukocytes, and levels of cytokine gene expression. In contrast to acute stress, chronic stress was immunosuppressive. Chronic exposure to corticosterone, or acute exposure to dexamethasone significantly suppressed skin DTH. These results suggest that during acute stress, endogenous stress hormones enhance skin immunity by increasing leukocyte trafficking and cytokine gene expression at the site of antigen entry. While these results are discussed from a mechanistic and clinical relevance perspective, it is acknowledged that much work remains to be done to elucidate the precise mechanisms mediating these bi-directional effects of stress and stress hormones and their clinical ramifications.

Local administration of cytostatic drugs enhances delayed-type hypersensitivity to Sendai virus in mice

Cytostatic drugs known to exert immunomodulatory activity were studied in a local treatment protocol as to their effect on development of cell-mediated immunity to Sendai virus in mice. Strong enhancement of delayed-type hypersensitivity (DTH) was observed after treatment with the active cyclophosphamide (CY) derivative Z 7557 or the plant alkaloid VP-16 at the site of sensitization. Optimal enhancement of DTH was obtained after priming with subimmunogenic doses of Sendai virus and treatment with low, virtually nontoxic drug doses. Immunoenhancement is ascribed to elimination of CY-sensitive suppressor cells from the draining lymph nodes. The relevance of our findings with respect to protective immunity is discussed.

Immunomodulation by altered gastrointestinal tract flora. The effects of orally administered, killed Staphylococcus epidermidis, Candida, and Pseudomonas on systemic immune responses.

We investigated the role of microbial products from the gastrointestinal tract in the pathogenesis of immunosuppression in critical illness. Rats were gavaged daily for three weeks with killed Staphylococcus epidermidis, Candida, and Pseudomonas--organisms frequently isolated from the upper gastrointestinal tract of the critically ill patient; nonspecific immunity was measured by the delayed-type hypersensitivity (DTH) response. Gavage with either Pseudomonas or Candida resulted in significant suppression of DTH responses, while gavage with S epidermidis produced modest enhancement of DTH. Animals given Pseudomonas demonstrated impaired ability to localize a subcutaneous Staphylococcus aureus challenge and decreased IgM but enhanced anti-Pseudomonas IgA and IgG responses after intraperitoneal immunization. Pulmonary bacterial clearance at seven days was normal. These findings suggest that gut colonization with Candida or Pseudomonas may contribute to impairment of cell-mediated immunity in the critically ill patient.

Conditioned effects of cyclophosphamide. Enhancement of delayed-type hypersensitivity in the mouse.

Conditioned effects of cyclophosphamide. Enhancement of delayed-type hypersensitivity in the mouse.

Induction of delayed-type hypersensitivity to Leishmania major and the concomitant acceleration of disease development in progressive murine cutaneous leishmaniasis.

BALB/c mice injected intradermally with 10(5) or higher doses of formaldehyde-fixed promastigotes (FFP) of Leishmania major developed strong delayed-type hypersensitivity (DTH) to leishmanial antigens injected into the hind footpad 3 to 10 days later. The DTH peaked 15 to 18 h after footpad injection and disappeared by 48 h. This specific DTH correlated with the homing of 51Cr-labeled syngeneic bone marrow cells and the infiltration of proliferating cells to the site of antigen administration. Spleen cells from FFP-sensitized mice also gave significant proliferative response to FFP in vitro. The DTH was adoptively transferable by Lyt-1+2-L3T4+ T cells and was H-2 restricted. DTH could be substantially enhanced by pretreatment with cyclophosphamide or pertussigen. Such DTH enhancement was accompanied by concomitant exacerbation of disease progression after L. major infection. Mice injected intravenously with FFP developed substantial immunity to cutaneous leishmaniasis but specifically suppressed DTH reactivity. Treatment of mice with pertussigen before intravenous immunization, however, abolished the protection and reversed the suppression of DTH. These results therefore demonstrate that the early-appearing type of DTH is not involved in host protection but that it actually facilitates disease progression in cutaneous leishmaniasis. Further evidence, which also shows the nonspecific nature of this disease exacerbation, is provided by local cell transfer experiments. Splenic T cells from mice sensitized to keyhole limpet hemocyanin or FFP induced significantly larger lesions compared with normal T cells when they were transferred into the footpad together with specific antigen and L. major promastigotes.

Factors influencing the enhancement of delayed-type hypersensitivity to ovalbumin by cyclosporin A in the guinea pig: possible role of suppressor cells.

Enhancement of delayed-type hypersensitivity (DTH) reactions to ovalbumin (OVA) was demonstrated in guinea pigs given a single, high dose of cyclosporin A (CsA) intraperitoneally, 2 days before immunization. Courses of oral CsA, commencing at the time of immunization and lasting until day 4 or 13 also resulted in augmented DTH responses at days 14 and 28, respectively. However, the enhancing protocol (CsA; day 0-4) did not significantly affect circulating anti-OVA antibody titres. The capacity to express increased DTH could be adoptively transferred to naive recipients by systemic injection pooled spleen and peritoneal exudate cells. Moreover, the expression of augmented responses was inhibited by transfer of cells from normally immunized donors. Although augmentation of DTH was accompanied by increased lymphocyte proliferative responses to OVA in vitro, there was no similar effect on T cell responses to polyclonal mitogens. The data support the view that augmentation of DTH by CsA is attributable to suppressor cell dysfunction, but that this is unlikely to be a non-specific suppressor cell impairment.

Cellular cooperation in the expression of murine delayed-type hypersensitivity (DTH): 1. A normal accessory cell population enhances DTH produced by immune peritoneal exudate T lymphocytes

A murine system for local passive transfer of delayed-type hypersensitivity (DTH) has recently been described. It was determined that untreated and T-lymphocyte-enriched (nylon-wool-nonadherent) fractions of peritoneal exudate (PE) cells from immunized donors could be transferred with soluble antigen to normal recipient footpads to efficiently produce a local DTH response. Untreated spleen or lymph node (LN) cell populations were strikingly less capable in this regard. It is now reported that addition of normal untreated PE cell populations to immune T-enriched PE cells markedly enhanced the DTH response transferred by the latter. Specific swelling was dose dependent with respect to each cell type. Removal of T lymphocytes from the normal PE cell population did not affect its enhancement of DTH. By cotransfer of 1 × 10 7 normal PE cells, significant specific swelling was obtained using 1–3 × 10 5 T-enriched immune PE cells. This represented a three- to seven-fold reduction in the requirement for the latter cell type. This scheme of DTH enhancement was employed to evaluate the mechanisms for decreased capability of immune LN and spleen for DTH transfer when compared to PE. No evidence was found that either adherent or nonadherent suppressor cells are operative at the time of DTH expression. Cotransfer of a DTH-enhancing population failed to equalize DTH expression by LN and spleen with that of PE. It is concluded that DTH effector-T-cell activity is enriched in immune peritoneal exudate and that non-T-cell population(s) from that source actively enhance DTH expression.

Augmentation of delayed-type hypersensitivity reactions to ovalbumin by cyclophosphamide in the mouse: Strain variability, antigen specificity and nature of the suppressor cell

The use of complete Freund's adjuvant (CFA) or pretreatment with cyclophosphamide (Cy) resulted in augmented delayed-type hypersensitivity (DTH) reactions in mice immunized with ovalbumin (OVA) and challenged 8 days later with heat-aggregated antigen. In 7 different strains, the degree of enhancement of DTH ranged from 2.5 to 4.2-fold with CFA and from 1.2 to 3.6-fold with Cy. More intense 24-h reactions (increased 3.3-8.8-fold) were elicited by use of both Cy and CFA. The effect of Cy was reversed by systemic transfer (at the time of immunization) of immune spleen cells, but not by cells treated with anti-Thy-1.2 and complement. The capacity of the suppressor T cells to reverse the potentiating effect of the drug was antigen-specific.

Enhancement of delayed-type hypersensitivity and induction of interferon by the lipophilic agents DDA and CP-20,961

The lipophilic amines dimethyl dioctadecyl ammonium bromide (DDA) and N,N-dioctadecyl- N′, N′-bis(2-hydroxyethyl)propanediamine (CP-20,961) are compared on their capacities to induce interferon, nonspecific protection to viral infection, and enhancement of delayed-type hypersensitivity (DH). DDA, a well-known adjuvant for the induction of DH is a moderate interferon inducer like CP-20,961. On the other hand, CP-20,961, a known interferon inducer and resembling in structure DDA, is shown to enhance DH to inactivated Semliki Forest virus (SFV). Nonspecific protection to challenge with a lethal dose of either SFV or encephalomyocarditis (EMC) virus was induced on injection of both compounds.

Modulation of the immune system in the mouse. 1. Drug administration prior to antigen sensitization.

A model has been developed in the mouse in which methylated bovine serum albumin (MBSA) and sheep red blood cells have been used to produce delayed type hypersensitivity (DTH) and humoral immunity (HI), respectively. The time course between antigen sensitization and challenge was chosen such that cyclophosphamide (CY), administered prior to sensitization, produced DTH enhancement and also produced suppression of HI when administered at high doses. A number of other drugs have been examined in this model but only CY-like drugs, viz. alkylating agents, produced similar DTH enhancement. DTH was suppressed in a few cases. The HI response was enhanced or suppressed by a number of unrelated drugs. CY was the only alkylating agent to suppress the antibody titre. The mechanisms for these drug effects are uncertain. However, a number of drugs elicit effects on either DTH or HI which suggests there is selective removal of certain cell populations rather than non-specific cytotoxicity. Possible theories for the effects observed are discussed. These studies also suggest that CY possesses unique properties in this particular model.

Modulation of the immune system in the mouse. 2. Drug administration following antigen sensitization.

The effects of administering drugs 2 days after antigen sensitization in a combined model of delayed type hypersensitivity (DTH) and humoral immunity (HI) in the mouse are described and compared with a previous study in which drugs were administered prior to sensitization using the same model. Low doses of cyclophosphamide (CY) administered after antigen sensitization were found to enhance DTH to methylated bovine serum albumin (MBSA). In addition, high doses of this drug suppressed DTH for 3 days after challenge, and then enhanced the response for a further 5 days. The HI response to sheep red blood cells (SRBC) was simultaneously suppressed by a range of CY doses. A number of different drug types demonstrated a similar pattern of dose-dependent DTH effects. However, unlike the previous study DTH enhancement was not confined to the alkylating agents. Either DTH enhancement or suppression was also observed after treatment with a number of other drugs. HI was suppressed by several drugs, in particular by a number of alkylating agents and anti-metabolites. Selective effects on one or other limbs of the immune response were observed. The mode(s) of action of drugs active in the present study are uncertain but possible explanations are discussed in terms of the elimination and subsequent recovery of specific cell populations.

Induction of autoreactive T lymphocytes and their suppressor cells by cyclophosphamide

THERE is increasing evidence of potentially autoreactive cells in normal animals1,2. Expression of autoimmunity, however, may not arise as a primary effector cell abnormality but more possibly as the consequence of a defect in a thymus-dependent control mechanism3–5. Cyclophosphamide (CY) enhances the capacity of mice to give T cell-dependent responses, such as delayed type hypersensitivity (DTH) induced by sheep red blood cells6. It was suggested that B cells were the targets of CY action7 and that the enhanced DTH reaction was due to inhibition of antibody production and consequently to lack of the antigen–antibody complexes responsible for inhibition of DTH. Askenase et al.8 presented evidence for an alternative explanation for enhancement of DTH by CY. They demonstrated augmented DTH to sheep red blood cells with doses of CY which did not influence antibody responses, suggesting the existence of CY-sensitive suppressor cells. Similar conclusions have been drawn by Roellinghoff et al.9, who observed induction of cytotoxic lymphocytes in mice to hapten-conjugated syngeneic cells after treatment with CY. We report here that injection of normal mice with CY results in manifestation of autoreactive T lymphocytes followed by the appearance of suppressor cells counteracting autoreactivity.