Monoclonal anti-T Cell Research Articles

Transplantation of Enriched and Purged Peripheral Blood Progenitor Cells From a Single Apheresis Product in Patients With Non-Hodgkin's Lymphoma

High-dose chemotherapy with or without radiotherapy followed by autologous transplantation of hematopoietic progenitor cells is an effective treatment for patients with high-risk or relapsed non-Hodgkin's lymphoma. Chemotherapy and/or hematopoietic growth factors have been used to mobilize progenitor cells in the peripheral blood for transplantation. However, the mobilized blood cell products have been found to be frequently contaminated with tumor cells, and techniques have not been developed to purge tumor cells from these products. In addition, the minimum number of hematopoietic progenitor cells required for engraftment has not yet been fully elucidated. We treated 21 patients with a single infusion of cyclophosphamide (4 g/m2) followed by daily administration of granulocyte colony-stimulating factor (G-CSF). After recovery of the white blood cell count, a single 3-hour apheresis collection was performed. The apheresis product was then applied to a discontinuous Percoll gradient. The low-density fractions resulting from this separation procedure were enriched for CD34+ progenitor cells (total cell yield, 19.5%; CD34+ cell recovery, 81.2%). These enriched cellular products were treated with a panel of anti-B cell or anti-T cell monoclonal antibodies and complement in an effort to remove residual tumor cells. After treatment of the patient with myeloablative therapies, the enriched and purged cells were reinfused. Hematologic recovery was rapid, with median neutrophil engraftment in 10 days [absolute neutrophil count (ANC), greater than 0.5 x 10(9)/L] and 11 days (ANC, greater than 1.0 x 10(9)/L). Median platelet transfusion independence required 13 days. The rapidity of multilineage engraftment correlated with the number of CD34+ cells per kilogram that were infused. Patients who received more than 2 x 10(6) CD34+ cells per kilogram had rapid hematologic engraftment, whereas those patients transplanted with less than 2 x 10(6) CD34+ cells per kilogram had slower platelet recovery. Modeling studies using a lymphoma cell line with a t(14; 18) chromosomal translocation demonstrated the successful removal of tumor cells assayed using the polymerase chain reaction (PCR) after the processing and purging. Four of the 21 patients had PCR-detectable lymphoma cells in the bone marrow and peripheral blood; however, the enriched and purged blood products reinfused in all four did not contain detectable tumor cells.(ABSTRACT TRUNCATED AT 400 WORDS)

INDEFINITE ALLOGRAFT SURVIVAL INDUCED BY THE COMBINATION OF MULTIPLE DONOR-SPECIFIC TRANSFUSIONS, CYCLOSPORINE, AND AN ANTI-T CELL MONOCLONAL ANTIBODY IN A PROTOCOL RELEVANT TO CADAVERIC ORGAN TRANSPLANTATION THE IMPORTANCE OF PROLONGED POSTTRANSPLANT CYCLOSPORINE COVERAGE

A single donor specific transfusion (DST) 24 hr pretransplant and Cyclosporine (CsA) are synergistic in inducing prolongation of allograft survival. This synergy was further potentiated by giving 3 additional DSTs at weekly intervals posttransplant, or by the additional administration of a small dose of the anti-T cell monoclonal antibody OX-52 24 hr pretransplant to the above protocol. This study determined whether indefinite survival can be achieved in the ACI-to-Lewis heart allograft model by combining an anti-T cell monoclonal antibody, 4 DSTs given once 24 hr pretransplant and weekly 3 times posttransplant, and CsA. CsA was given daily for 28 days or 60 days posttransplant. The dose consisted of CsA 5 mg/kg/day starting 24 hr pretransplant and continuing until day 7 posttransplant and followed by 2.5 mg/kg/day for either 21 more days (total CsA days = 28 days) or 53 days (total CsA days = 60 days). The anti-T cell monoclonal antibody OX52 (200 micrograms i.v.) was given once 24 hr pretransplant. DST (1 ml) was given 24 hr pretransplant and on days 7, 14, and 21 posttransplant. Low-dose CsA for 28 days never induced indefinite allograft survival. CsA for 60 days, however, resulted in the occasional indefinite allograft survival (> 1 year) even in this difficult model. The addition of DST 24 hr pre- and posttransplant on days 7, 14, and 21 to both 28 days and 60 days of CsA further prolonged allograft survival. The best survival was seen in the group given DST 24 hr pre- and on days 7, 14, and 21 posttransplant and 60 days of CsA, with all the allografts surviving beyond 100 days of CsA, with all the allografts surviving beyond 100 days and more than 50% surviving indefinitely (> 1 year). The addition of a single dose of OX52 24 hr pretransplant to the multiple DSTs and 28 days of CsA protocol significantly improved indefinite allograft survival but its influence was less dramatic when given to the multiple DSTs and 60 days of CsA protocol. This beneficial interaction between CsA, DST, and an anti-T cell MoAb offers a clinically applicable protocol for both living donor and cadaveric organ transplantation in inducing donor-specific hyporesponsiveness, and further investigations are warranted.

Insulin-Dependent Diabetes Mellitus as an Autoimmune Disease

IDDM is unquestionably an autoimmune disease, as reflected by the presence of beta-cell-reactive autoantibodies and T cells, T cell-mediated transfer of the disease in nondiabetic mice, rats, and humans, and disease sensitivity to immunosuppressive therapy. T cells are predominantly, if not exclusively, involved in creating the islet lesions that lead to beta-cell atrophy after a stage of reversible inflammation. A full understanding of the disease pathogenesis will require a better definition of the nature of the triggering and target autoantigen(s) and of the effector mechanisms (cytokines, cytotoxic cells?). Much less information is available on the etiology than on the pathogenesis. Genetic factors are mandatory and the involvement of predisposition genes (HLA and non-HLA) is now being unravelled. The modulatory role of environmental factors is demonstrated by the high disease discordance rate in identical twins and by experimental data showing positive and negative modulation of the disease by a number of agents, notably infectious agents and food constituents. It is not clear, however, whether a given environmental factor, e.g. a precise virus or a cow's milk component, plays a real etiological role in a selected genetic background. IDDM thus appears as a multifactorial disease. It is not known, however, whether all factors intervene concomitantly in a given individual or separately in subsets of patients, explaining the clinical heterogeneity of the disease. The mechanisms underlying the loss of tolerance to self beta-cell autoantigen(s) are still unknown. Defective intrathymic negative selection of autoantigen-specific autoreactive T cell clones is unlikely. Breakdown of T cell anergy could occur according to various mechanisms, including aberrant expression of MHC molecules and molecular mimicry. Defective suppressor T cell function, perhaps related to TH1/TH2 imbalance, probably intervenes by amplifying the anti-beta-cell autoimmune response whatever its triggering mechanism. Before putative etiological agents are identified, one must base immunotherapy on nonantigen-specific agents. Results recently obtained in NOD mice indicate that the goal of nontoxic long-lasting immune protection from the disease is feasible if treatment is started early enough. In some cases (anti-T cell monoclonal antibodies), it appears that specific unresponsiveness can be induced. This double strategy (early intervention, tolerance induction) is the main challenge for immunodiabetologists.(ABSTRACT TRUNCATED AT 400 WORDS)

Evidence of CD4+ regulatory T cells in the non-obese diabetic male mouse

The NOD mouse, which shows many features of human IDDM, is extensively used to evaluate the role of T lymphocytes in the pathogenesis of autoimmune diabetes. The development of diabetes in this model appears to be controlled by a finely tuned immunoregulatory balance between autoaggressive T cells and regulatory immune phenomena, the disruption of which may result in destruction of insulin-secreting cells. The absolute requirement of sublethal irradiation to permit transfer of the disease to non-diabetic adult syngeneic mice provides indirect evidence for the presence of regulatory T cells in non-diabetic NOD mice. We have previously reported that the reconstitution of irradiated recipients by CD4 + T cells from nondiabetic female NOD mice blocks the transfer of diabetes by spleen cells from diabetic donors. We now report evidence that anti-CD4 monoclonal antibodies can substitute for irradiation in rendering adult NOD male mice susceptible to diabetes transfer by diabetogenic spleen cells. Efficient diabetes transfer can be achieved in non-irradiated adult NOD recipients provided they are thymectomized and CD4 + T-cell depleted prior to the transfer. The role of thymectomy is to limit T cell regeneration after anti-T cell monoclonal antibody challenge. Our data confirm that regulatory CD4 + T-cells, which efficiently counterbalance diabetogenic cells, are present in adult NOD male animals.

Evidence of CD4+ regulatory T cells in the non-obese diabetic male mouse.

The NOD mouse, which shows many features of human IDDM, is extensively used to evaluate the role of T lymphocytes in the pathogenesis of autoimmune diabetes. The development of diabetes in this model appears to be controlled by a finely tuned immunoregulatory balance between autoaggressive T cells and regulatory immune phenomena, the disruption of which may result in destruction of insulin-secreting cells. The absolute requirement of sublethal irradiation to permit transfer of the disease to non-diabetic adult syngeneic mice provides indirect evidence for the presence of regulatory T cells in non-diabetic NOD mice. We have previously reported that the reconstitution of irradiated recipients by CD4+ T cells from nondiabetic female NOD mice blocks the transfer of diabetes by spleen cells from diabetic donors. We now report evidence that anti-CD4 monoclonal antibodies can substitute for irradiation in rendering adult NOD male mice susceptible to diabetes transfer by diabetogenic spleen cells. Efficient diabetes transfer can be achieved in non-irradiated adult NOD recipients provided they are thymectomized and CD4+ T-cell depleted prior to the transfer. The role of thymectomy is to limit T cell regeneration after anti-T cell monoclonal antibody challenge. Our data confirm that regulatory CD4+ T-cells, which efficiently counterbalance diabetogenic cells, are present in adult NOD male animals.

Transfer of protection to murine typhoid conferred by L-form Salmonella typhimurium in dependence of cooperation between L form-adopted macrophages and L form-induced Lyt-2+ T cells.

The effector cells responsible for protection to Salmonella typhimurium in C3H/HeJ mice, conferred by L-form S. typhimurium, were determined by cell transfer test. Nonfractionated spleen cells from 6-week immune mice but not from 24-week immune animals transferred anti-S. typhimurium immunity. Treatment with anti-macrophage antiserum and complement most effectively abolished protective capacity in 6-week immune cells, while anti-T cell monoclonal antibody plus complement reduced it to a lesser extent. However, adoptive protection was achieved only by transfer of immune macrophages along with Lyt-2+ T cells selected from 6-week immune spleen cells. These Lyt-2+ T cells were cytotoxic to Kupffer cells from C3H/HeJ mice which had been infected 48 hr previously and from the mice which had been immunized 1 week previously, but not to the cells from 6-week immune mice and from normal animals. Moreover, protective capacity in immune macrophages seemed to be correlated to the degree of colonization by the L forms, and the inability to transfer immunity of 24-week immune spleen cells may be due to the decrease in the L form-colonization. These results suggest that cooperation between the L form-colonized macrophages and L form-induced cytotoxic Lyt-2+ T cells contributes to anti-S. typhimurium immunity, and might imply the immunological difference between the 6-week immune phagocytes and the cells at an early stage of infection or immunization.

Therapeutic Effects of Monoclonal Antibodies to αβTCR but Not to CD4 on Collagen-Induced Arthritis in the Rat

The role which T cells play in the pathogenesis of the collagen-induced arthritis (CIA) model is not yet fully understood. Although CIA is most likely dependent on the activity of class II-restricted CD4 + T cells, only prophylactic but not therapeutic anti-CD4 treatments have been successful. The lack of therapeutically effective anti-T cell monoclonal antibody treatments has questioned the importance of T cells in ongoing CIA. However, recently we found that ongoing CIA in DA rats induced with homologous CII can be suppressed by injections with an anti-αβCR antibody. Having a CIA model where ongoing disease was clearly dependent on T cells, we addressed in the present work whether also an anti-CD4 treatment could suppress ongoing arthritis in this model. Although no CD4 hi lymph node cells were seen after an anti-CD4 injection, the arthritis was suppressed only after treatment at immunization but not after treatment just before onset of disease. In comparison, the anti-TCR treatment at the time of onset was clearly suppressive even though a large fraction of the T cells was not depleted. This indicates that the different outcome of the anti-TCR and anti-CD4 treatment was not due to a different capacity to deplete T cells in vivo.

Induction of tolerance to heart allografts in rats using posttransplant total lymphoid irradiation and anti-T cell antibodies.

This study examined whether posttransplant anti-T cell monoclonal or polyclonal antibody therapy could provide a window of treatment to allow posttransplant total lymphoid irradiation (TLI) to induce tolerance. These experiments were conducted in a high responder strain combination of an ACI cardiac allograft into a Lewis rat. In this situation, treatment with antibody or posttransplant TLI alone is insufficient to induce tolerance, while similar treatments alone have been shown to induce tolerance in low responder strains. The affects of three anti-T cell therapies were compared: anti-CD4 mAb therapy, anti-CD3 mAb, and rabbit antithymocyte globulin (RATG). None of these antibody therapies alone prolonged graft survival indefinitely. Combining anti-CD4 therapy with posttransplant TLI markedly delayed rejection but failed to induce long-term graft survival. Tolerance could be induced by a combination of anti-pan T cell antibody (anti-CD3) and TLI, and, all grafts survived beyond 100 days. RATG failed to prevent graft rejection when used alone or in combination with TLI. However, posttransplant therapy with a combination of RATG, TLI, and single-donor blood transfusion resulted in graft survival beyond 100 days. Recipients bearing long-term donor grafts rejected third-party (PVG) grafts within 2 weeks. Low density donor bone marrow cells used instead of a blood transfusion did not facilitate tolerance. The results indicate that monoclonal or polyclonal anti-pan T cell antibodies, TLI, and a donor blood cell infusion function synergistically in facilitating tolerance to allografts in the posttransplant period.

Murine anti-mouse T cell monoclonal antibodies elicit anti-antibodies in mice: intra-species immunization model for estimating potential patient sensitization against humanized anti-T cell antibodies.

Humanization of immunosuppressive anti-T cell monoclonal antibodies (mAb) raises the question as to how completely it helps to avoid formation of neutralizing anti-antibodies (anti-Ab) in patients. To get more information on intra-species sensitization against anti-T cell mAb, we produced two immunosuppressive mouse IgG2a anti-mouse Thy-1.2 mAb (MmT1 and MmT5) in AKR/J mice and measured the potential of MmT1 to elicit inhibitory anti-Ab in AKR/J (H-2k), C57BL/6 (H-2b), congenic B10.BR (H-2k) and DBA/2 (H-2d) mice. After one injection once weekly for 4 weeks of 5 micrograms MmT1 (200 micrograms/kg) in C57BL/6 mice, without the use of any adjuvants, high concentrations of anti-Ab directed against MmT1 (300 micrograms/ml) and MmT5 (100 micrograms/ml) were measured by enzyme-linked immunosorbent assay. Similar concentrations of anti-Ab were found in immunized DBA/2 and less in B10.BR mice. No syngeneic anti-Ab could be produced in AKR/J. From the C57BL/6 mice, we raised anti-MmT1+, MmT5- idiotype (IDIO1) and anti-MmT1+, MmT5+ allotype (ALLO1) mAb. An in vivo test system was adapted to measure the inhibitory effects of circulating poly- or monoclonal anti-Ab. It revealed a reduction of in vivo depletion capacity not only of the sensitizing mAb (MmT1), but also of another anti-Thy-1.2 mAb (MmT5), with identical allotype but different idiotype. From this we conclude that intra-species immunization following injection of anti-T cell mAb can produce high titer inhibitory anti-idiotype and anti-allotype antibodies. Implications for hyperchimeric or fully human anti-T cell mAb are discussed.

Failure to demonstrate suppressor T cell activity in the lamina propria of Hymenolepis diminuta-infected rats.

The aim of this study was to examine whether there was an increase of suppressor T cells, relative to helper T cells, in the intestinal lamina propria of Hymenolepis diminuta-infected rats, a condition which might allow the parasite to survive for the life-span of the host. Lamina propria cells were isolated by enzymatic procedure. All lymphocytes were passed over nylon wool columns in order to remove B cells; then the T cells were purified by a panning technique using monoclonal anti-T cell antibodies. Changes in the mitogen-stimulated synthesis of IgM, IgG and IgA by normal peripheral blood indicator lymphocytes, as measured by sandwich ELISA, was used as an index of help and/or suppression. No significant suppressor T cell activity was observed in the cultures, either from control or infected intestine.

Immunosuppression promotes CNS remyelination in chronic virus‐induced demyelinating disease

Immunosuppression using cyclophosphamide or anti-T cell monoclonal antibodies (mAbs) directed at CD4 or CD8 promoted remyelination of CNS axons in the spinal cords of mice infected chronically with Theiler's virus. Treatment with a mAb directed at class II major histocompatibility gene products did not increase the extent of CNS remyelination. Following immunosuppressive treatment, quantitative morphometry revealed a five- to sevenfold increase in new myelin synthesis. Proliferating nervous system cells were identified at the edges of remyelinated lesions by their incorporation of [3H]thymidine. CNS remyelination occurred in mice depleted of selected subsets of T lymphocytes despite the local persistence of viral antigen. These findings indicate that CNS remyelination occurs as a normal consequence of primary myelin injury, but factors associated with immune T cells somehow impair remyelination. Interference with the function of immune T cells enhances CNS remyelination by oligodendrocytes. Similar depletion of immune T cells may allow for enhanced remyelination in the CNS of patients with chronic multiple sclerosis.

Splenocyte response to T cell-derived cytokines in thymectomized Xenopus

A miniaturized, “hanging-drop” bioassay reveals that splenocytes from earlythymectomized (Tx) Xenopus can respond (by enhanced thymidine incorporation) to thymicdependent “cytokines” generated in PHA- or alloantigen-stimulated cultures. Preliminary evidence, using fluorescence activated cell sorting, indicates that surface IgM − splenocytes, rather than sIgM + cells, from Tx toads are sensitive to the crude, splenocyte-derived, active supernatants. Although these responsive cells display residual, but low, reactivity to PHA, their thymus independence is suggested by flow cytometric observations using the anti-T cell monoclonal antibody XT-1. The development of “T-like” cells in Tx Xenopus is discussed.

Monoclonal anti-T cell (T12) antibody treatment of graft-versus-host disease in severe combined immunodeficiency: targeting of antibody and activation of complement on CD8 + cytotoxic T cell surfaces

We report a 6-month-old male child with severe combined immunodeficiency who received an unirradiated blood transfusion and developed acute, severe graft-versus-host disease (GVHD), for which he received monoclonal anti-T cell (anti-T12) antibody treatment. The GVHD was manifested by a confluent maculopapular rash and increased liver function tests and was documented by skin biopsy. Separation of peripheral blood mononuclear cells forming rosettes with sheep red blood cells revealed engrafted T cells having the nonrelated HLA type of the blood donor. The patient was treated with intravenous monoclonal anti-T12 in a dose of 0.3 mg/kg/day for 5 days. An in vivo effect of the anti-T12 was suggested by clinical improvement of his skin rash and return of the liver transaminases to the normal range. Moreover, human complement components, activated C3 and C4, were detected by fluorescence microscopy on the surfaces of the engrafted CD8 + lymphocytes on the skin biopsy specimens. Also, with a biotin-avadin assay, the presence of the anti-T12 was detected on these same cells. These studies document not only the in vivo targeting of monoclonal anti-T12 antibody to cytotoxic T cells producing GVHD but also the activation of complement on these cells.

Purification and stabilization of a poorly soluble mouse IgG3 monoclonal antibody

The anti-T cell monoclonal antibody (Mab) RIV9 (mouse IgG3, κ) has been developed for clinical use in the treatment of allograft rejection. In order to obtain a clinical grade Mab preparation, RIV9 was purified from cell culture supernatants by protein A affinity and anion exchange chromatography. Reasonable yields of highly purified product could only be obtained if stabilising compounds were added and Tween 80 was used in all stages of the purification process. Prior to anion exchange chromatography, dextran sulphate (MW 5000) was added to keep the Mab in solution. Many other additives were tested but did not solubilise RIV9 under the low salt strength conditions required for ion exchange chromatography. The complex character of the solubility-determining factors was demonstrated by the influence of buffer composition, buffer concentration, pH, and sodium chloride concentration on the solubility of RIV9.

Screening for growth hormone gene deletions in patients with isolated growth hormone deficiency

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Treatment of Guillain-Barre syndrome with anti-T cell monoclonal antibodies.

Three patients with acute Guillain-Barré syndrome were treated early in the course of the disease with OKT3, an anti-T-cell monoclonal antibody. Each patient developed acute lymphopenia with specific depletion of T3-positive lymphocytes. Two patients had continued progression of clinical deficits for eight and 14 days. This suggests that T lymphocytes may not be essential for progression of Guillain-Barré syndrome at this stage of the disease.

Prevention of lethal graft-vs.-host disease by a single low dose injection of anti-T cell monoclonal antibody to the allograft recipients.

We investigated the capacity of monoclonal antibody (mAb) treatment to prevent graft-vs.-host disease (GVHD) in lethally irradiated, allogeneically reconstituted mice, employing anti-T cell (subset) mAb and a fully allogeneic strain combination. In this strain combination, purified CD4+ cells were able to induce a lethal GVH reaction, whereas purified CD8+ cells were not. In the same strain combination, a single intraperitoneal injection of IgG2b anti-Thy-1 mAb, one day after reconstitution, caused a dose-dependent improvement of the survival. A single injection of a dose as low as 12.5 micrograms per mouse was already effective. Intravenous and intraperitoneal administration of the mAb appeared equally effective. For effective prevention of GVHD the treatment could be postponed until the 4th day after transplantation, but treatment delayed until day 6 was no longer effective. Treatment with IgG2b mAb specific for either helper or cytotoxic T cells also led to improvement of GVHD and survival, but was less effective than treatment with anti-Thy-1 mAb. Clinically, there was a difference in the effectiveness of anti-CD4 and anti-CD8 treatment, since symptoms of GVHD started earlier in the anti-CD8 treated group and the survival was better in the anti-CD4 treated group. These results press for prospective clinical studies employing anti-T cell mAb treatment early after allogeneic bone marrow transplantation, especially in HLA mismatched cases.

The role of OKT3 in clinical transplantation

OKT3 is a murine monoclonal anti-T cell antibody that is directed to CD3, a five-chain molecular complex found in association with the T cell receptor for antigen. OKT3 was the first monoclonal antibody to be used in organ transplantation and during the past 10 years there has been extensive experience of its use both for preventing and treating rejection in organ transplantation. OKT3 blocks T cell function by modulating CD3 and the T cell receptor from the T cell surface. A reaction to OKT3 results from cytokines released when OKT3 first reacts with T cells. This reaction is generally mild but can be severe. First rejections following kidney transplantation are reversed in approximately 95% of cases. Steroid-resistant rejections are also susceptible to OKT3 but in only approximately 75% of cases. When used for prophylaxis, OKT3 completely blocks rejection in 95% of patients and significantly delays the onset of rejection in those who do reject. Antibodies to OKT3 are produced in approximately 75% of patients receiving it. However, seldom are the antibodies to OKT3 present in high titer and only in those cases is successful re-use of OKT3 prevented. As is the case with all potent immunosuppressive drugs, the use of OKT3 is associated with increased viral, specifically cytomegalovirus, infections. However, it appears that reduction of concomitant immunosuppression decreases the incidence of severe infections. Unquestionably, OKT3 has been a useful addition to the immunosuppression used for organ transplantation. In addition, its use has stimulated research on other monoclonal antibodies for use in organ transplantation.

Interferon gamma plays a critical role in induced cell death of effector T cell: a possible third mechanism of self-tolerance.

We have used anti-T cell monoclonal antibodies (mAbs) as mimic ligands to study the effects of T cell receptor (TCR) ligation of cloned T helper type 1 cells in the presence or absence of accessory cells. Our results demonstrate that ligation of the TCR in the absence of accessory cells rapidly induces cell death. Cell death can be prevented by addition of spleen adherent cells, leading to strong clonal expansion. Induced cell death is inhibited by cyclosporin A and by anti-interferon gamma (IFN-gamma), and is restored by adding exogenous recombinant IFN-gamma to cyclosporin A-treated cells. These results demonstrate that IFN-gamma plays a critical role in cell death induced by anti-TCR mAbs in the absence of costimulatory cells. We propose that induced cell death of active effector T cells provides a third mechanism of tolerance in addition to intrathymic deletion of developing autoreactive T cells and peripheral inactivation of mature, naive T cells.

A unique subset of normal murine CD4+ T cells lacking Thy‐1 is expanded in a murine retrovirus‐induced immunodeficiency syndrome, MAIDS

Some strains of mice inoculated with LP-BM5 murine leukemia virus (MuLV) develop a syndrome, termed mouse acquired immunodeficiency syndrome (MAIDS), characterized by progressive lymphoproliferation and profound immunodeficiency. LP-BM5 MuLV is a virus mixture that contains ecotropic (eco) and mink cell focus-induced MuLV and a defective genome that is the proximal cause of disease. Flow cytometry analyses of spleen and lymph nodes from susceptible C57BL/6 mice infected with this virus mixture revealed the presence in spleen and peripheral lymph nodes of a previously unrecognized subset of CD4+CD3+ T cells that are Thy-1-. The frequency of these cells increased with progression of disease, eventually comprising between 30% and 50% of all CD4+ cells. Infection of A/J mice, a strain which is genetically resistant to development of MAIDS, did not induce an increase of this T cell population, indicating that infection with the virus mixture was insufficient to induce its proliferation. A central role for the defective virus in this process was suggested by the finding that C57BL/6 mice infected with LP-BM5 eco alone did not have increased frequencies of Thy-1-CD4+ cells in spleen. Studies of spleen and peripheral lymph node cells from normal mice demonstrated the presence of Thy-1-CD4+ cells at frequencies of 1%-2%. Studies using two anti-T cell monoclonal antibodies, SM6C10 and SM3G11, that define four CD4+ subsets showed that Thy-1-CD4+ T cells from normal and infected mice were present only in the 6C10- subsets.