Murine sarcoma virus (MSV)-induced tumors in mice. I. Distribution of MSV-immune cytolytic T lymphocytes in vivo

In vivo evaluation of the effects of interleukins 2, 4 and 7 on enhancing the immunotherapeutic efficacy of anti-tumor cytotoxic T lymphocytes

The draining lymph nodes of mice injected with viable syngeneic tumor cells contain lymphoid cells capable of generating anti-tumor cytotoxic T lymphocytes (CTL) during a 4-day in vitro culture period and intravenous infusion of relatively low numbers of these CTL can effectively eliminate a challenge of fibrosarcoma cells at distal skin sites that would otherwise result in the death of the host. Using this model system the effects of addition of interleukin (IL) 2, IL4 or IL7 to the culture medium on the therapeutic efficacy of the anti-tumor CTL generated was investigated. In this regard, IL7 was found to be the most potent of the cytokines tested. Addition of IL7 either alone, or in combination with low doses of IL2, resulted in the generation of CTL with significantly (6-8-fold) enhanced therapeutic efficacy in vivo. Anti-tumor effector cells generated in the presence of IL7 were also found to be at least 4-fold more effective at eliminating established tumors than CTL generated in medium alone. Of the other cytokines tested, addition of IL2 resulted in elevated CTL activity in vitro, but only a modest (approximately 2-3-fold) enhancement of the therapeutic efficacy in vivo. Addition of IL4, either alone or in combination with IL2, also resulted in the generation of effector cells with enhanced tumoricidal activity in vitro and yet the therapeutic efficacy of these cells was decreased compared to that of CTL generated in medium alone. These observations indicate that (a) inclusion of IL7 in the medium in which tumor-reactive T cells are cultured can markedly enhance the immunotherapeutic efficacy of the resulting effector cell populations; and (b) in vitro assays of tumoricidal activity cannot be used as a reliable predictor of therapeutic efficacy in vivo.

Emergence of a dominant cytotoxic T lymphocyte antitumor effector from tumor-infiltrating cells in the anterior chamber of the eye

Previous studies in mice revealed that resolving intraocular tumors (UV5C25 fibrosarcoma) were infiltrated with mononuclear cells and invoked potent systemic delayed-type hypersensitivity responses without nonspecific tissue destruction. The present study characterized the tumor-infiltrating lymphocyte (TIL) population and established its role as the mediator of specific intraocular tumor rejection. This was accomplished by (a) isolating TIL from resolving intraocular tumors; (b) identifying characteristic surface markers on TIL; and (c) demonstrating in vitro and in vivo antitumor functions. Fluorescence-activated cell sorter analysis of TIL showed 33.4% Thyl+, 19.8% CD8+, 11.1% CD4+, 17.2% MAC-1+, 10.4% F4/80+, and 7.7% B220+. Functional studies indicated that TIL were directly cytolytic for UV5C25 tumor cells. Additionally a tumor-necrosis-factor(TNF)-sensitive cell line (WEHI 164.1) was lysed on cocultivation with TIL, whereas UV5C25 tumor cells were insensitive to lysis by TNF. Precursor CTL analysis demonstrated a high frequency (1/251) of tumor-specific precursors and a low frequency of alloresponsive cells in the TIL population. In vivo analysis by a Winn-type assay demonstrated that only TIL could effect tumor resolution in immunosuppressed hosts. These results demonstrate that although CD4+ T cells and macrophages were present and TNF activity was detected in the TIL population, there was no evidence for nonspecific tissue destruction within the eye. Therefore, this pattern of intraocular tumor rejection is mediated by a lymphocyte population expressing cell-surface phenotypes and functional characteristics of conventional cytotoxic T lymphocytes. Moreover, the results suggest that a regulatory mechanism within the eye allows for the emergence of one dominant antitumor effector (CTL) while controlling a more destructive mechanism (delayed-type hypersensitivity).

Mechanisms of tumour rejection in the murine DBA/2-SL2 concomitant immunity system

The mechanisms of rejection of secondary tumours were studied in concomitant immunity using transplants of standard-size solid SL2 tumours in syngeneic DBA/2 mice. In such a system primary tumor implants are not rejected, in contrast to secondary tumour implants. The second tumour was mainly rejected 2-4 days after implantation. Both primary tumour and secondary tumour implants (which are rejected) contained hardly any lymphocyte infiltrate, whereas, 2-4 days after implantation they contained 40%-50% macrophages, which were cytotoxic in vitro. Transfer (s.c.) of these tumours to naive mice showed that cellular infiltrates in the secondary implants did not always cause tumour rejection. Serum collected on day 4 after implantation of the secondary tumour was cytotoxic to SL2 tumour cells in vitro, whereas serum from mice with only primary implants was not cytotoxic on day 4 after implantation. Preliminary characterization of this cytotoxic factor showed that it was heat-labile, as cytotoxicity disappeared after 30 min at 56 degrees C, the molecular mass of the factor was higher than 100 kDa, and it was not IgG. We hypothesize that secondary tumours in the DBA/2-SL2 concomitant immunity system are rejected mainly between 2 and 4 days after implantation as a result of the combined action of cytotoxic serum and the presence of 40%-50% cytotoxic macrophages. The primary tumour is not rejected at 2-4 days after implantation as there is no cytotoxic factor.

T-cell responses at the host: tumour interface

The evidence considered here reinforces the conclusion that T-cell responses to tumours involve complex cellular interactions. An attempt to summarize some of these interactions is shown. This emphasizes that not only are the interactions between the effector cell populations complicated, but that the target cell surface is also subject to variation and modification as a result of the immune response. A feature that also emerges from these studies is that most cells apparently responding to or infiltrating a tumour do not necessarily participate in its destruction, and it is in this area that experimental tumour systems have particular value. This also perhaps explains the preoccupation of experimentalists with the identification of 'the' effector cell crucial to tumour rejection. However, there is heterogeneity between systems in terms of the type of rejection response induced, but a logical basis for this heterogeneity is not established. If experimental studies could define the nature of the immune response generated by a tumour in the context of the biological features of the tumour itself, this could lead to the prediction of the immunogenicity and potential for induction of a rejection response for that tumor. Clearly, experimental tumour systems do not provide an exact reflection of the situation with human tumours. However, they may provide systems that illuminate particular aspects of the human response, and give precedents to guide the interpretation of data derived from human systems. This form of assessment is still at an early stage, but developments in the experimental field should provide a framework for the development and exploitation of T-cell responses to tumours.

Lymphocyte migration during the development of regional lymph node anergy in experimental tumor growth

The development of lymph node anergy in Wistar rats to growing Walker carcinoma 256 was studied in vitro using the 51Cr-release cytotoxicity assay. Cell-mediated cytotoxicity to the tumor peaked in draining lymph nodes 11 days after tumor transplantation. By 14 days, the regional lymph node had become anergic to the tumor at a time when cell-mediated cytotoxicity was still increasing in the more distal contralateral lymph node. Lymphocyte migration into resting, cytotoxic, and anergic lymph nodes was analyzed to determine if altered cell migration into the regional lymph node was associated with the development of anergy. Lymphocyte migration was found to be enhanced in both cytotoxic and anergic regional lymph nodes of tumor-bearing animals. It is concluded that lymph node anergy in this experimental tumor system is not related to changes in lymphocyte migration patterns; rather, it is the result of alterations in the microenvironment of the lymph node which prevents the expression of cytotoxic effector cells.

Specificity studies on cytolytic T lymphocytes directed against murine leukemia virus-induced tumors. Analysis of monoclonal cytolytic T lymphocytes

The specificities of cloned cytolytic T lymphocytes (CTL) were studied for the analysis of CTL populations generated against murine leukemia viruses (MuLV) in H-2 congenic BALB/c (H-2d) and BALB.B (H-2b) mice. In particular, CTL generated in response to tumors induced by Gross MuLV and Friend MuLV were studied; these tumors expressed virus-induced antigens that do not cross-react and that can be distinguished from each other. The systematic study of 92 CTL clones clearly indicated that MuLV-immune CTL were highly heterogeneous with respect to both the intensities of target cell lysis that they mediated and to their specificity of recognition of MuLV-induced tumor target cells. Various categories of CTL clones were identified, ranging from CTL clones tht were tightly H-2 restricted and specific for the immunizing tumor to CTL clones that displayed no discernible patterns of specificity and that attacked a large number of different target cells. In addition, the surface markers of these cloned CTL were defined, and the best conditions for their prolonged maintenance in culture were determined. The present data indicate that future efforts in the definition of target antigens recognized by tumor-specific CTL should be performed with monoclonal lymphocytes.

Quantitation and clonal isolation of cytolytic T lymphocyte precursors selectively infiltrating murine sarcoma virus-induced tumors

A limiting dilution mixed leukocyte-tumor cell microculture system was used to quantitate cytolytic T lymphocytes and their precursors (CTL-P), which infiltrate tumors induced by injection of Moloney sarcoma-leukemia virus (MSV-MoLV) complex into C57BL/6 mice. Leukocyte populations obtained from tumors collected on day 10 after virus injection were found to contain significantly higher frequencies of operationally defined (tumor-specific) CTL-P than either peripheral blood leukocytes (PBL) or spleen cells from the same animals. When these frequencies were normalized according to the content of Lyt-2+ T cells in each tissue, average CTL-P frequencies were found to be 1/9 in tumor-infiltrating cells vs. 1/41 in PBL. These results directly demonstrate selective accumulation of CTL-P in the tumor mass. A number of clonal isolates obtained from tumor-infiltrating leukocyte populations were expanded and studied for cytolytic activity and specificity. Of 11 isolates, 10 were found to have high cytolytic activity, leading to 50% lysis of the syngeneic MoLV-derived tumor target cells in 3.5 h at lymphocyte:target cell ratios ranging from 0.5:1 to 3.2:1. Furthermore, five randomly selected clones showed H-2 restriction by their selective lytic activity against MoLV-derived syngeneic MBL-2 target cells and their lack of activity against either MoLV-derived allogeneic (LSTRA) tumor cells or against syngeneic (EL4) or allogeneic (P815) target cells unrelated to MoLV.

In vitro demonstration of in situ autologous tumour-cell cytotoxicity in MSV-induced tumours in A/SN mice

Moloney sarcoma-virus (MSV)-induced tumours in A/Sn mice have been dispersed with collagenase and DNase 8-15 days after virus inoculation, and both "sarcoma" and inflammatory cells separated by sedimentation velocity and adherence techniques. The isolated "sarcoma" cells had the morphological characteristics of atypical cells (i.e. cytoplasmic blebbing, vacuolization and prominent nucleoli) and were easily adapted to in vitro growth. As few as 2 x 10(3) of these cells inoculated i.m. produced new tumours within 8 days of injection in both syngeneic and allogeneic mice. Also, cell-free supernatant from "sarcoma"-cell cultures produced tumours, indicating that the successful transplantation of the "sarcoma" cells was probably due to production of infective virus. Cells cytotoxic in vitro against the "sarcoma" cells were present within both spleen and tumour of the tumour donors, but not in the spleens of normal mice. The cytotoxicity was specific against virus-infected cells, since in a mixture of virus-positive (gp 70) and virus-negative cells, positive cells were removed while negative cells were not affected, as measured by a visual cytotoxicity assay using immunostaining. Although T cells could be isolated from the MSV-induced tumours, these cells did not appear to mediate the cytotoxicity detected against the MSV "sarcoma" cells. These results suggest that early MSV infections might be sensitive to cytotoxic mechanisms distinct from those reported with established MLV- or MSV-induced tumour lines.

Use of hybridoma-resident variant cell lines to study H-2Db/FMR-specific cytotoxic T cells

An H-2Db heterozygous tumor cell line and a variant subclone bearing a mutant gene product were used to analyze the H-2Db specificity of cytotoxic T lymphocytes (CTL) generated during a Moloney murine sarcoma virus (MSV) infection. When the mutant cells were used as targets for MSV-specific CTL, the amount of cell lysis, compared with that seen with the nonmutant parental cells, was drastically decreased. However, cells of the mutant clones remained susceptible to allogeneic CTL specific for the nonmutant H-2Db molecule. The mutant cells also did not differ from the parent cells in their level of viral antigen expression. Biochemically the parental and mutant molecules were similar but not identical. The data indicate that minor alterations of the H-2 antigens caused by somatic mutation may prevent virus-infected cells from being recognized as targets by CTL.

Antigenic specificity of the cytolytic T lymphocyte response to murine sarcoma virus-induced tumors. III. Characterization of cytolytic T lymphocyte clones specific for Moloney leukemia virus-associated cell surface antigens

Cytolytic T lymphocyte (CTL) clones specific for Moloney leukemia virus (MoLV)-derived tumor cells were generated by placing limiting numbers of C57BL/6 responder cells into mixed leukocyte-tumor cell microcultures. Under appropriate conditions (presence of stimulating tumor cells, accessory cells, and T cell growth factor), such cloned CTL could readily be expanded to provide large numbers of homogeneous, highly cytolytic CTL populations for further characterization. Using four target-cell types, three specificity patterns were observed: one reactive with the syngeneic MoLV-derived tumor only, one cross-reactive with an allogeneic MoLV-derived tumor, and one cross-reactive with normal allogeneic cells. Subclones derived from these three types of clones exhibited a high degree of stability in terms of lytic activity and specificity over a 4-mo period of observation. Three clones analyzed by flow cytofluorometry using monoclonal antibodies were all found to be of the Lyt-1+2+ phenotype. Furthermore, lysis of target cells by all of six clones tested was inhibited by anti-H-2Db (but not by anti-H-2Kb) monoclonal antibodies, demonstrating H-2Db-restriction at the clonal level.

Active immunization against murine radiation-induced leukemia using a sarcoma virus pseudotype

Immunization of C57BL/Ka mice against the radiation leukemia virus (Rad LV) markedly reduced their susceptibility to development of leukemia after X-radiation. The active immunogen used consisted of a sarcoma virus pseudotype of Rad LV which induces regressing sarcomas in young adult mice. These immunized animals were challenged with four weekly 160-rad X-ray exposures. The appearance of lymphomas and leukemias resulting from such irradiation was delayed and their incidence significantly decreased. These results indicate that oncornavirus immunization may be used to reduce the incidence of murine leukemias which are triggered by a physical agent.

Development and persistence of cytolytic T lymphocytes in regressing or progressing Moloney sarcomas

Intratumoral T lymphocytes were recovered sequentially after induction of regressing or progressing Moloney sarcomas in BALB/c mice and were assayed quantitatively for their ability to kill specifically the tumor (MSC) cells used for induction. The cytolytic activities of the two lymphocyte populations described two distinct biphasic kinetic profiles that were similar in amplitude and duration but separated from each other by 4-6 days. In progressing neoplasm, there was a rapidly occuring accumulation of T lymphocytes highly cytolytic for MSC cells. This response, however, was not sustained and disappeared in association with the onset of unchecked tumor growth. In contrast, T lymphocyte cytolytic activity developed more slowly in regressing sarcomas and attained peak levels coincident with the beginning of tumor regression. Similar changes in cytolytic activity characterized T lymphocytes in lymph nodes draining tumors. When cultured in vitro for 4 days, non-cytotoxic T lymphocytes from regional lymph nodes draining progressing sarcomas regained very high levels of cytolytic activity. Such restitution was diminished, however, if MSC cell lysates, macrophages or macrophages fed MSC cell lysates were present during the culture period. These experiments provided presumptive evidence that T lymphocyte-mediated cytolytic activity was lost in progressively growing Moloney sarcomas as a consequence of suppression in vivo of the genesis and/or functional expression of cytolytic T lymphocytes, perhaps by macrophages and/or soluble tumor antigen.