Virus-Infected Mice. Alabama the host before antibody transfer. This suggests

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1 INFECTION AND IMMUNITY, Aug. 1980, p Vol. 29, No /80/ /08$02.00/0 Lymphocyte Reactivity Contributes to Protection Conferred by Specific Antibody Passively Transferred to Herpes Simplex Virus-Infected Mice JOHN E. OAKES,* WILLIAM B. DAVIS, JOHN A. TAYLOR, AND WILLIAM A. WEPPNERt Department ofmicrobiology and Immunology, College ofmedicine, University of South Alabama, Mobile, Alabama Passively acquired immunity to herpes simplex virus (HSV) was studied in antithymocyte serum (ATS)-treated mice and athymic nude mice to determine whether immunocompetent lymphocytes contribute to the protection observed after transfer of HSV-specific antibody to infected animals. Mice were given three intraperitoneal injections of 0.1 ml of ATS at 24-h intervals. This treatment reduced concanavalin A and lipopolysaccharide stimulation of lymphocytes harvested from these animals by 90% when compared with the stimulation of lymphocytes harvested from untreated animals. It was found that intraperitoneal injection of 0.5 ml of specific antibody 8 h after corneal HSV type 1 infection or subcutaneous HSV type 2 infection did not protect ATS-treated animals from virus infection. Specific antibody passively transferred to ATS-treated animals 8 and 120 h postinfection also failed to protect lymphocyte-depleted animals from HSV. However, ATS-treated animals were protected from HSV infection by passively acquired antibody when lymphocytes harvested from these animals regained 80% of their ability to be stimulated with concanavalin A and lipopolysaccharide. It was also found that specific antibody conferred protection to nude mice infected with HSV only if they were first reconstituted with syngeneic thymus cells 48 h before infection. The results suggest that both antiviral antibody and thymus-derived lymphocytes contribute to the recovery of HSV-infected hosts after passive immunization. Antibody specific for herpes simplex virus (HSV) has been shown to protect susceptible mice from virus infection when passively transferred to infected animals within 48 h postinfection (4, 18, 19). Previously, we have demonstrated that sublethal total body irradiation of the host before antibody transfer abolishes the protective effects of passive antibody in HSVinfected animals (9, 27). These experiments suggested that a population of cellular effectors in addition to specific antibody was needed for recovery of the host from HSV infection. However, these studies did not indicate the nature of the cells inhibited with irradiation, since sublethal total body irradiation is known to interfere with a number of immunological reactions involving effector cells (1). This study was initiated to determine whether the cellular effectors needed in recipients of antibody for protection against HSV infection are lymphocytes. To obtain animals deficient in lymphocyte populations, we used as the virusinfected host either mice treated with antithyt Present address: Riker Laboratories, 3M Center, St. Paul, MN mocyte serum (ATS) or mice congenitally lacking a thymus. The results of the study demonstrated that administration of antiviral antibody did not protect HSV-infected mice when immunocompetent lymphocytes capable of eliciting cell-mediated immunity were not present in the host before antibody transfer. This suggests that both passively acquired antiviral antibody and lymphocyte reactivity contribute to the recovery of HSV-infected hosts after passive immunization. MATERIALS AND METHODS Cells and viruses. Vero cells, obtained from Flow Laboratories, Inc., Rockville, Md., were maintained in disposable roller bottles (Bellco Glass Inc., Vineland, N.J.) in minimum essential medium supplemented with antibiotics, 10% fetal calf serum, and NaHCO3. HSV type 1 (HSV-1) (strain kos) and HSV-2 (strain 333) were obtained from Fred Rapp, Hershey, Pa. Virus stocks were prepared from plaque-purified virus. Monolayers of Vero cells were infected at a multiplicity of 0.1 plaque-forming unit per cell, and the virus was harvested from cells 28 h later by three cycles of freezing and thawing. The supernatant was clarified and assayed on Vero cells as previously described (27). Mice. Normal inbred 4- to 5-week-old BALB/c mice 642

2 VOL. 29, 1980 (NIH subline of Andervant strain) were obtained from the Leo Goodwin Institute, Ft. Lauderdale, Fla. Fourweek-old congenitally athymic nude (nu/nu) BALB/ c mice were obtained as specific pathogen-free animals from the Charles River Breeding Laboratory, Wilimington, Mass. The latter animals were housed in a barrier-sustained animal room with appropriate temperature and relative humidity. All food, water, bedding, and cages were autoclaved before use in the animal facility. Preparations and passive transfer ofanti-hsv hyperimmune sera. Previously we have shown that mouse anti-hsv serum prepared from mice undergoing a secondary antibody response is as effective in protecting recipients as rabbit hyperimmune serum (27). In this study, we used rabbit antisera because it was easier and cheaper to prepare. Rabbit anti-hsv- 1 and anti-hsv-2 hyperimmune sera were prepared by multiple intravenous inoculation of virus into 3- month-old New Zealand white rabbits as previously described (12, 25). Serum from individual rabbits was harvested by cardiac puncture, heat inactivated at 560C for 30 min, and stored at -70'C until use. The sera from several rabbits were pooled, and the neutralizing antibody titer was determined as previously described (33). The pooled anti-hsv-1 sera used in the experiments had a neutralizing titer of 1:1,600, whereas the pooled anti-hsv-2 sera had a neutralizing titer of 1:1,200. Animal inoculations and passive immunization. Normal BALB/c mice were infected with approximately 3 50% lethal doses of HSV-1 or HSV-2 as estimated by the method of Reed and Muench (34). This dose of virus was routinely found to kill 90% or more of the infected animals 10 to 12 days postinfection. For HSV-1 infection, mice were anesthetized with nembutol. HSV-1 (3.8 x 107 plaque-forming units) was then dropped onto the right eye after scarification by three trephine motions with a corneal trephine. The virus inoculum was then massaged into the eye with the eyelid. For HSV-2 infection, mice were infected subcutaneously in the right rear footpad with 104 plaque-forming units of HSV-2. After infection with either virus, mice were passively immunized intraperitoneally with the appropriate rabbit antisera 8 h after infection. Before passive transfer, antisera were diluted in phosphate-buffered saline to contain a neutralizing titer of approximately 1:150 as previously described (27). It was found that the susceptibility of nude BALB/ c mice to ocular and subcutaneous HSV infection did not differ significantly from normal mice. The lack of increased susceptibility of nude mice to HSV infection has also been noted by other investigators (21, 41) and may be due to the fact that athymic nude mice have macrophages which can be nonspecifically activated by antigens (21, 43) or to an increase in interferon production in these animals (41). Therefore, the same virus doses used to infect normal mice were also used to infect athymic nude mice. Reconstitution of athymic nude mice. Thymus tissue from 4-week-old BALB/c mice was minced in RPMI 1640 medium with 10% newborn calf serum and passed through gauze. The cells were washed twice in RPMI 1640 medium and adjusted to 2 x 109 viable PROTECTION CONFERRED BY ANTI-HSV ANTIBODY 643 cells per ml. The lymphocytes were inoculated intravenously into nude mice in a volume of 0.1 ml. Treatment of animals with ATS. Rabbit ATS was purchased from Microbiological Associates, Bethesda, Md. ATS-treated animals were given three intraperitoneal injections of 0.1 ml of ATS at 24-h intervals. Twenty-four hours later, they were injected with either HSV-1 or HSV-2. Lymphocyte transformation assay. The immune reactivity of spleen cell populations from normal, ATS-treated, and ATS-treated plus HSV-infected mice was demonstrated in the lymphocyte transformation assay (2). Spleens were aseptically removed and teased apart in Hanks balanced salt solution plus 0.5% dextran. After passage through a syringe fitted with a 26-gauge needle, the spleen cells were sedimented by centrifugation at 60 x g for 10 min and washed twice with Hanks balanced salt solution plus 0.5% dextran. The cells were finally suspended in RPMI 1640 supplemented with 25 mm HEPES (N-2-hydroxyethylpiperazine-N'-2-ethanesulfonic acid) buffer, 50,ug of penicillin and streptomycin per ml, 50 Mg of fungizone per ml, 2 mm glutamine, and 2.5% fetal calf serum. The cell concentration was adjusted to 107 cells per ml, and 100 volumes were aliquoted into triplicate wells of a Microtest II plate (Falcon Plastics, Inc., Oxnard, Calif.). Mouse splenocytes were found to be optimally stimulated by 5IMg of concanavalin A (ConA) per ml and 10 Mg of lipopolysaccharide (LPS) per ml. All experiments included controls of medium alone and spleen cells plus medium in the absence of mitogen. The complete cultures were incubated 72 h at 37 C under 5% CO2 before termination. During the last 18 to 24 h of incubation, the cultures were labeled with 1,uCi of [3H]thymidine per well (New England Nuclear Corp., Boston, Mass; specific activity, 6.7 Ci/mmol). The cells were harvested onto glass fiber filters (Reeves Angle Co., Englewood Cliffs, N.J.) with a cell harvester. The dried filters were counted in a Beckman LS-9000 liquid scintillation counter in toluene containing 0.3% 2,5-diphenyloxazole and 0.025% 1,4-bis-[2-(5-phenyloxazolyl)]benzene. The results were calculated as a stimulation index, which is the ratio of the mean counts per minute incorporated into lymphocytes in the presence of mitogen preparations to the mean counts per minute incorporated into lymphocytes not incubated with mitogen. The significance of the stimulation index of treated groups as compared with control groups was determined by the Student t test. Statistical analysis. The survivor trends of each experimental group of mice were transformed by negative exponential transformation (O = 0.1, T = 20) as described by Liddel (17). The means and variances of each group of mice were then calculated from the transformed data, and the level of significance between survivors in control groups and survivors in experimental groups was evaluated by Student's t test. For some experiments the level of significance was determined by the chi-square goodness of fit with Yates correction factor (42). RESULTS Reduction of lymphocyte stimulation and

3 644 OAKES ET AL. peripheral blood lymphocytes in ATStreated animals. ConA and LPS stimulation of lymphocytes harvested from ATS-treated animals was tested to determine the effects of ATS on lymphocyte responses in vivo. Stimulation responses to these mitogens were also measured in lymphocytes harvested from ATS-treated animals infected with HSV (Fig. 1). The response of lymphocytes to ConA and LPS in all ATStreated groups declined significantly after two ATS doses (P < 0.01). However, the maximum effect of ATS therapy was seen 4 days after the final ATS treatment, when the response of lymphocytes harvested from ATS-treated animals and ATS-treated animals infected with HSV declined by 90%. Stimulation responses were also measured in lymphocytes harvested from ATS-treated animals 21 days after ATS therapy to determine whether these lymphocytes had recovered from the effects of ATS treatment. These measurements indicated that lymphocytes from ATS-treated animals recovered 70% of their immune reactivity to ConA and LPS 21 days posttherapy. Small blood samples from five animals were collected and individually counted immediately before the start of ATS treatment and 2 days after final ATS treatment to determine the effects of ATS on the number of circulating lymphocytes. After differential counts on the samples, it was found that ATS treatment also caused a 55% reduction in the number of blood INFECT. IMMUN. lymphocytes. ATS abolished the protective effects of antibody. Once we had demonstrated that ATS therapy diminished the number of circulating lymphocytes and their immune reactivity, we next tested for the protective effects of antibody in lymphocyte-depleted animals. We found that 3 days of ATS therapy abolished the protective effects of specific antibody passively transferred to HSV-infected mice (Fig. 2). In this experiment, specific antibody protected control mice, whereas ATS-treated animals died from virus infection even though they had received identical quantities of virus-specific antibody (P < 0.01). Even though specific antibody did not protect ATS-treated animals from herpesvirus infection, antibody significantly (P < 0.02) delayed the onset of virus-induced death in HSV- 1-infected, ATS-treated animals from 4 to 8 days postinfection and the onset of virus-induced death in HSV-2-infected, ATS-treated animals from 10 to 14 days postinfection. Due to these results, we next tested the possibility that an additional antibody treatment would cause further delays in the progression of infection, perhaps leading to full recovery of lymphocyte-depleted animals challenged with HSV. In this experiment, ATS-treated mice infected with HSV-1 were given a second dose of antibody 5 days after the first antibody dose. This coincided with the time that mice normally produce anti-hsv-1 antibody after corneal in- 24 a so CON A a 0 NO ATS * +ATS A + ATS + HSV U 14 o NO an S W ATS A + an + HSV 60 w 9 so I'C ::: _ : : _ # ' g IFECTION DAYS IOFECTION 2 3 DAYS 9 21 FIG. 1. Stimulation index of spleen cells harvested from normal mice, ATS-treated mice, and mice treated with ATS and infected with HSV-1 after incubation in the presence of 5 /g of ConA or 10 ptg of LPS. There were three mice per group (each time point), and the points are the average ofthree separate experiments. The bars represent + standard error of the mean. A, _

4 VOL. 29, 1980 PROTECTION CONFERRED BY ANTI-HSV ANTIBODY 645 A. B. co 'Is LIs 0 z w w DAYS POSTINFECTION DNYS POSTINFECTION FIG. 2. Effect of ATS on antibody-dependent immunity to HSV infection. Groups of mice were given 3 intraperitoneal injections of 0.1 ml ofats or no treatment at 24-h intervals. Twenty-four hours after the last injection, animals were infected either on the right cornea with HSV-1 or subcutaneously in the right rear footpad with HSV-2. At 8 h postinfection, groups received either anti-hsv-i antibody or normal rabbit serum (NRS) (A) and either anti-hsv-2 antibody or NRS (B). There were 10 mice per group. The animals were observed for an additional 3 weeks, during which time there were no additional mortalities. fection (15). It was found that additional transfer of specific antibody to lymphocyte-depleted animals did not significantly influence the outcome of virus infection since mice receiving two antibody doses 5 days apart did not survive significantly longer than mice receiving one antibody dose 8 hours postinfection (Fig. 3). Antibody protected ATS-treated animals infected 21 days after ATS therapy. Since ATS eliminated the protective effects of antibody in vivo, it was of interest to determine whether specific antibody would protect ATStreated animals once lymphocytes harvested from these animals regained immunological responses to ConA and LPS. This question was answered by infecting groups of ATS-treated animals 9, 15, and 21 days after ATS treatment and intraperitoneally injecting antibody 8 h after virus inoculation (Table 1). It was found that recovery of the protective effects of passive antibody in ATS-treated animals coincided with the recovery of lymphocyte reactivity to ConA and LPS (Fig. 1). Specific antibody conferred protection to nude mice infected with HSV only when they were reconstituted with thymus cells. Virus-specific antibody was tested for its protective effects against HSV infection in congenitally athymic nude mice. Groups of normal or nude athymic BALB/c mice were infected corneally with HSV-1 or subcutaneously with HSV-2. Eight hours later, a portion of the HSV-1-infected animals was given anti-hsv-1-specific antibody and a portion of the HSV-2-infected animals was given anti-hsv-2-specific antibody (Fig. 4). It was found that specific antibody did not significantly protect athymic nude BALB/c animals from virus infection (P < 0.05). As an additional control in this experiment, we retested the protective effects of the antisera by transferring identical quantities of the appropriate pooled antibody to normal mice 8 h after HSV-1 or HSV-2 infection. All antibody-treated mice survived infection, as has been demonstrated earlier (Fig. 2 and 3), whereas 90% or more of untreated infected mice died (data not shown). Even though specific antibody did not influence the final outcome of virus infection in athymic nude mice, athymic nude mice given specific antibody survived significantly longer after HSV infection than did athymic nude mice given normal rabbit serum (P < 0.02). To examine the effects of thymus-derived lymphocytes on the immunity to HSV infection in passively immunized animals, we reconstituted nude mice with thymus cells from syngeneic donors. It has been shown that many T- dependent immune responses can be restored in athymic nude mice when they are reconstituted with thymus cells (13, 14, 20, 28, 30). After reconstitution, a significant number of mice receiving thymus cells and antibody survived HSV infection (P < 0.05) as compared with animals receiving antibody or thymus cells alone (Table 2). The results of this experiment suggest that the presence of antiviral antibody in HSV-in-

5 646 OAKES ET AL. INFECT. IMMUN. ATS TREATED * HSV-1 + NRS * HSV-1 X- Ah R knntinfo hoc tinn C 70 A HSV-1 + Ab 8 and 120 hrs 0 Postinfection > ~~~~~~~CONTROLS cc Z 40 w 9X HSV-1 + NRS en 50 HSV-1 + Ab DAYS POSTINFECTION FIG. 3. Mortality ofats-treated mice infected with HSV-1 after two injections of HSV-1-specific antibody 8 and 120 h postinfection. Groups of mice were given three intraperitoneal injections of 0.1 ml of ATS or no treatment at 24-h intervals. Twenty-four hours after the last ATS injection, animals were infected on the right cornea with HSV-1. Two groups received either normal rabbit serum (NRS) or anti-hsv-i antibody (Ab) 8 h postinfection, and one group received anti-hsv-i antibody 8 and 120 h postinfection. TABLE 1. Recovery of the protective effects of passively acquired anti-hsv-i antibody in ATS treated mice infected with HSV Time of HSV-1 in- No of survivors/ fection (days after No. of aninals' % Survivors ATS treatment) no. of aniials 9 1/ / /10 80 a Mice were given three intraperitoneal injections of 0.1 ml of ATS at 24-h intervals. At selected times after the final ATS injection, animals were infected on the right cornea with HSV-1. Eight hours after infection, animals received anti-hsv-1 antibody. The results were tabulated three weeks after infection. There was no change in mortalities during three additional weeks of observation. fected hosts is not sufficient to initiate recovery without the participation of thymus-derived lymphocytes. DISCUSSION T-cells (24, 31), macrophages (21, 23, 36), and antibody (4, 9, 19, 27) all have been shown to be involved in host resistance to HSV infection. Even though each of these immunological mechanisms may be found to initiate recovery when they are studied individually, in many circumstances host resistance to HSV may involve cooperation between two or more of these components. Cooperation between antibody and a Cn' 70 0 HSV2A > IS DAYS POSTINFECTION FIG. 4. Susceptibility ofathymic nude mice to HSV infection after passive transfer of anti-hsv antibody (Ab). Mice were infected either on the right cornea with HSV-I or subcutaneously in the right rear footpad with HSV-2. At 8 hpostinfection, groups received either normal rabbit serum (NRS), anti-hsv-i antibody, or anti-hsv-2 antibody. There were 10 mice per group. Mice were observed for an additional 3 weeks, during which time there were no additional mortalities. cellular component of the immune system in passively immunized animals was noted in earlier experiments when it was found that sublethal total body irradiation abolished the antiviral activity of antibody administered to in-

6 VOL. 29, 1980 TABLE 2. Protective effects ofpassively acquired anti-hsv-i antibody in reconstituted athymic nude mice infected with HSV-1 a breatment Treatmentb No. vors/no, of survi- Of % Survivr animals PROTECTION CONFERRED BY ANTI-HSV ANTIBODY 647 vors NRS + HSV-1 0/10 0 Ab + HSV-1 2/10 20 Thymus cells + HSV-1 4/10 40 Ab + thymus cells + HSV-lC 8/10 80 a Athymic nude mice received 2 x 108 thymus cells intravenously 48 h before they were infected on the right cornea with HSV-1. Eight hours after infection, animals received anti-hsv-1 antibody. The results were collected 3 weeks after infection. bab, Anti-HSV-1 antibody; NRS, normal rabbit serum. 'Differs from NRS + HSV-1 controls (P < 0.01). fected mice (9, 27). In this report, the results suggest that the cellular effectors required in recipients of specific antibody are lymphocytes. Evidence for interaction between lymphocytes and antibody in recovery of HSV-infected mice was initially obtained from experiments in which it was found that animals treated with ATS were no longer protected from virus infection in the presence of neutralizing antibody. However, the increased susceptibility to virus infection noted in these experiments could have been due to the effects of ATS on B-cells as well as T-cells since ATS significantly reduced biological activity in both of these lymphocyte populations. Thus, the inability of B-cells to produce additional antiviral antibodies to contribute to the protective effects of passively administered rabbit antibody in ATS-treated animals could have accounted for their increase in susceptibility to virus infection. We tested for this possibility by giving infected ATS-treated mice an additional injection of antiviral antibody at a time postinfection when humoral immunity is first detected in HSV-1-infected mice after ocular infection (15). Therefore, additional quantities of specific antibody were made available to host defense mechanisms to replace any ATS-induced damage to B-cell synthesis of antiviral antibody. The results of this experiment showed that transfer of additional antiviral antibody to ATS-treated mice infected with HSV had no effect on the outcome of virus infection. This suggests that the failure of B-cells to make additional antibody in HSV-infected, ATS-treated mice does not result in their increased susceptibility to HSV. It is also possible that ATS therapy may have had a deleterious effect on other cells of the immune system besides lymphocytes. One important component in host defenses against HSV infection which has been reported to be sensitive to ATS is the macrophage (5, 8). These cells are known to reduce spread of HSV within the host by either inhibiting virus replication within infected cells or by inhibiting virus replication by extrinsic mechanisms (23, 39). Macrophages are also important effector cells in antibody-dependent cellular cytotoxicity, and they are known to synthesize certain complement components which can contribute to antiviral immunity by interacting with HSV-infected cells coated with antibody (16, 31, 37, 38). Since macrophages are so important in host resistance to HSV, interference with their functions may have contributed to the inability of specific antibody to protect ATS-treated mice from HSV. For these reasons, we examined the protective effects of specific antibody in HSV-infected athymic nude mice. These animals were selected for our study because they lack any measurable T-cell responses but possess abundant numbers of immunologically active macrophages (3). In addition, nude mice possess immunocompetent B-cells and as well as natural killer cells (40). Therefore, these animals can respond to HSV infection with most of the immunological mechanisms available to normal mice except for cell-mediated responses as elicited by thymus-derived lymphocytes. When the protective effects of antibody were examined in athymic mice, it was found that specific antibody did not protect these animals from HSV infection. The results of this experiment suggest that T-cells are the lymphocyte subpopulation needed for recovery from HSV infection in recipients of specific antibody. Although our data have not excluded the possibility that lymphocytes contribute to host protection in the presence of specific antibody by acting as effector cells in antibody-dependent cell cytotoxicity or by some other mechanism (A. Bukacek, M. Roper, and C. M. Balch, Fed. Proc. 38:1161, 1979; H. D. Kay and D. A. Horwitz, Fed. Proc. 38:1435, 1978), these cells probably cooperate with antibody in passively immunized animals by mediating one or more of the cellular immune responses known to be important in limiting herpesvirus infections (7, 10, 11, 22, 24, 26, 32). By examination of Fig. 2 and 4, it can be seen that even though specific antibody did not influence the final outcome of virus infection in either ATS-treated mice or athymic mice, these animals survived significantly longer after HSV infection than did ATS-treatment mice and athymic nude mice not given specific antibody. Thus, specific antibody appears to be slowing down the rate at which virus spreads from sites of infection. Since it requires approximately 5 to 6 days for a detectable cell-mediated response to be generated after HSV infection (22, 35), we

7 648 OAKES ET AL. believe that specific antibody probably cooperates with cell-mediated immunity in immunocompetent mice by reducing the rate of virus spread until specifically sensitized lymphocytes appear. Once these cells are present, they then interact with virus-infected tissues to eliminate any virus which may have escaped antibody. The inability of ATS-treated mice and athymic mice to generate specific cell-mediated immunity would account for the limited amount of antiviral protection seen in these animals after transfer of specific antibody. A requirement for cell-mediated immunity in passively immunized animals would also explain why athymic nude mice were protected from HSV by specific antibody after thymus cell reconstitution. It has been shown that thymus cells can restore many T-cell-dependent functions in athymic nude mice (13, 14, 20, 28, 30). Therefore, reconstitution of nude mice with thymus cells probably provided a source of immunocompetent lymphocytes to mediate T-cell reactions in these animals. Once T-cell responses were present, passive transfer of specific antibody protected animals from virus infection. Unfortunately, our results do not indicate the precise mechanisms by which specific antibody reduces the rate of virus spread in HSV infected animals during the time that cell-mediated responses are not present. However, several mechanisms could account for the observed effects of antibody on virus spread in vivo. These include neutralization of newly synthesized virus in vivo, and the interaction of antibody with infected cells to mediate ADCC or complement-mediated cell lysis (6, 16, 29, 37, 38). Regardless of the antibody-dependent mechanisms involved in reducing virus spread, the results of this study demonstrate that the presence of antiviral antibody in HSV-infected hosts is not sufficient to initiate recovery without the participation of thymus-derived lymphocytes. ACKNOWLEDGMENTS We thank J. H. Coggin, Jr., and Robert Lausch for their continuing interest and support. The technical assistance of Richard Semmes is gratefully acknowledged. This work was supported by Public Health Service grant Rl01 A from the National Institute of Allergy and Infectious Diseases. INFECT. IMMUN. LITERATURE CITED 1. Anderson, R. E Ionizing radiation and the immune response. Adv. Immunol. 24: Anderson, J., G. Moller, and 0. Sjoberg Selective induction of DNA synthesis in T and B lymphocytes. Cell. Immunol. 4: Bach, J. F., M. A. Bach, and J. Charriere In M. Selegman, J. C. Preud'homme, and F. M. Kourlisky (ed.), Membrane receptors of lymphocytes, p North Holland Publishing Co., Amsterdam. 4. 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