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Impact of sex, MHC, and age of recipients on the therapeutic effect of transferred leukocytes from cancer-resistant SR/CR mice
© Stehle et al; licensee BioMed Central Ltd. 2009
Received: 19 June 2009
Accepted: 15 September 2009
Published: 15 September 2009
Spontaneous Regression/Complete Resistant (SR/CR) mice are resistant to cancer through a mechanism that is mediated entirely by leukocytes of innate immunity. Transfer of leukocytes from SR/CR mice can confer cancer resistance in wild-type (WT) recipients in both preventative and therapeutic settings. In the current studies, we investigated factors that may impact the efficacy and functionality of SR/CR donor leukocytes in recipients.
In sex-mismatched transfers, functionality of female donor leukocytes was not affected in male recipients. In contrast, male donor leukocytes were greatly affected in the female recipients. In MHC-mismatches, recipients of different MHC backgrounds, or mice of different strains, showed a greater negative impact on donor leukocytes than sex-mismatches. The negative effects of sex-mismatch and MHC-mismatch on donor leukocytes were additive. Old donor leukocytes performed worse than young donor leukocytes in all settings including in young recipients. Young recipients were not able to revive the declining function of old donor leukocytes. However, the function of young donor leukocytes declined gradually in old recipients, suggesting that an aged environment may contain factors that are deleterious to cellular functions. The irradiation of donor leukocytes prior to transfers had a profound suppressive effect on donor leukocyte functions, possibly as a result of impaired transcription. The cryopreserving of donor leukocytes in liquid nitrogen had no apparent effect on donor leukocyte functions, except for a small loss of cell number after revival from freezing.
Despite the functional suppression of donor leukocytes in sex- and MHC-mismatched recipients, as well as old recipients, there was a therapeutic time period during the initial few weeks during which donor leukocytes were functional before their eventual rejection or functional decline. The eventual rejection of donor leukocytes will likely prevent donor leukocyte engraftment which would help minimize the risk of transfusion-associated graft-versus-host disease. Therefore, using leukocytes from healthy donors with high anti-cancer activity may be a feasible therapeutic concept for treating malignant diseases.
Spontaneous regression/complete resistant (SR/CR) mice are a line of cancer-resistant mice that are capable of resisting large doses of transplanted lethal cancer cells [1–3]. The basis for this powerful resistance to cancer cells is leukocytes that are capable of detecting, infiltrating, and killing cancer cells within a few hours of exposure . The major component of this anti-cancer response resides in granulocyte, monocyte, and natural killer cell fractions that constitute innate cellular immunity [1, 2]. The adoptive transfer of donor leukocytes from SR/CR mice can confer protection against future exposures to cancer cells, as well as the elimination of established malignancies without any further manipulation in cancer-sensitive wild-type (WT) recipient mice .
These prior studies suggest that it may be possible to develop a similar leukocyte transfer platform for humans if we can identify humans with anti-cancer activity similar to the SR/CR mice to serve as leukocyte donors. However, the prior adoptive transfers of donor leukocytes in these mice were considerably different from what would be necessary in human cancer patients. The leukocyte transfers in mice were done in MHC-matched and relatively young recipients . In the potential human setting, the circumstance could be vastly different.
First, it is not known how much of an effect a sex mismatch would have on the donor leukocytes. It is possible for a donor leukocyte treatment to involve donors that are a different sex than the recipient. While female donor leukocytes in a male recipient are not expected to cause substantial problems, it is reasonable to expect some issues with male donor leukocytes in female recipients because of the existence of unique proteins associated with y-chromosome gene expression . However, it is unclear how much of an impact this incompatibility would have on the functionality of male donor leukocytes, and if this mismatch is enough to render the donor leukocytes ineffective in the recipients.
Second, in the human setting, a long-term engraftment of donor leukocytes, especially donor T lymphocytes, should be avoided in order to minimize the possibility of transfusion-associated graft-versus-host diseases (TA-GVHD) . The long-term engraftment of donor leukocytes in immunocompetent individuals is usually caused by an incomplete but near match of human lymphocyte antigens (HLA) between the donor leukocytes and the recipients, such as donor leukocytes from blood relatives [6, 7]. One way to help avoid long-term engraftment is by transferring donor leukocytes into immunocompetent recipients that have been selected based on a complete HLA mismatch. The complete HLA-mismatch between donors and recipients should cause the donor leukocytes to be rejected in several weeks to several months. Our prior mouse transfer experiments have shown that the donor leukocytes can work in a rapid manner over the course of several days, or 2-3 weeks in the MHC-matched background. In the setting of a human leukocyte transfer, a complete HLA-mismatched scenario would minimize the risk for TA-GVHD in an immunocompetent individual due to the timely rejection of the donor cells before the occurrence of TA-GVHD. However, it is unknown whether there is an initial therapeutic window before the donor leukocytes are eventually rejected due to the HLA-mismatch, in which the donor leukocytes may still offer an effective time period in which the donor leukocytes are functional.
Third, the transfers in the prior studies were done primarily with young mice as the recipients and donors . The scenario in the human setting would most likely be much more complicated. Since there is extensive immune dysfunction that occurs as one ages [8, 9], it is assumed that younger individuals would have better immune cell function and, therefore, make better donors. Even if younger individuals were used as donors, the leukocytes would be transferred into cancer patients who consist mainly of an older population . Based on prior results, it is thought that the aged cellular environment may contain circulating factors that are inhibitory to the functions of younger cells . However, it is currently unknown how much of an impact the older recipient environment would have on the functionality of younger donor leukocytes.
In this paper, we describe the results of studies that determine the impact of sex, MHC, and age on the therapeutic effects of SR/CR donor leukocytes in a lethal cancer model.
Results and discussion
Transferred anti-cancer activity in MHC-and sex-mismatched recipients
Transferred anti-cancer activity in recipients of different ages
Effect of irradiation on the function of transferred leukocytes
The irradiation of cells can be used to help minimize the risk of GVHD in adoptive immunotherapy settings . The irradiation irreversibly damages the DNA of the leukocytes, preventing their cell division without killing the cells. However, it is unclear if this process may have an immediate affect on the anticancer killing activity of the leukocytes. To evaluate this in our model, leukocytes of the spleen were harvested from C57BL/6 SR/CR mice, pooled, and then split into two groups. One group was irradiated (25Gy), while the other served as the nonirradiated control group. Each group was transferred to C57BL/6 WT mice and challenged 24 hours later with 1 × 106S180. The nonirradiated controls resulted in 100% survival, while the irradiated group displayed a reduction to only 38% survival (Figure 5). Overall, irradiation negatively impacted, but did not abolish the cancer killing activity of the SR/CR leukocytes.
Effect of cryopreservation on the function of donor leukocytes
Cryopreservation is a common practice for storing cells [13, 14]. Most of the time the viability of stored cells is minimally affected by freezing and thawing, it is not clear whether a specific functionality, such as the anticancer activity of SR/CR donor leukocytes, can be affected by cryopreservation. Leukocytes were harvested from SR/CR donor mice, pooled and divided into two groups. One was given directly to WT mice as a non-frozen control, while the other cells were frozen for approximately 30 days. Then, these cells were thawed and given to another group of WT mice. There were 6 mice used for the post-cryopreservation due to the loss of a tube of cells during the retrieval process. By using 6 for the group, then the number of cells given per recipient was closer to the control recipients' numbers (within 2 × 106 leukocytes). Both groups of WT mice were challenged with 1 × 106 S180 cells 24 hours after the designated adoptive transfers. Even with the slightly reduced number of leukocytes delivered after cryopreservation, both groups showed identical survival (100%) after a challenge with S180 (Figure 5). Therefore cryopreservation appeared to have no negative impact on the cancer killing activity of the SR/CR leukocytes.
It has long been theorized that clinically significant cancer is caused by a defective immune system which results in a loss of activity capable of removing cancer cells that are continuously being generated . However, adoptive transfer of innate leukocytes from donors with a validated high level of cancer-killing activity to cancer patients for therapeutic purposes is a new concept. In comparison, the major difference is that most conventional cancer immunotherapies try to stimulate a possibly damaged immune system of cancer patients, whereas this new concept attempts to supplement the damaged components of the immune system with ones with enhanced activity. Prior studies in SR/CR mice suggested that such a cancer surveillance system can be mediated by leukocytes of the innate immune system [1, 2]. Their powerful immune system is capable of eradicating a large number of malignant cells that otherwise would be lethal. It may be possible to develop similar therapeutic concepts in the human setting. The studies described in this paper investigated the factors that may have substantial impact on the efficacy of donor leukocytes therapeutically.
Our studies indicated that male recipients had no apparent inhibitory effect on the female donor leukocyte functionality. On the other hand, transfers of male donor leukocytes provoked a leukocyte rejection mechanism in female recipients to a considerable degree. This observation is consistent with the concept that y-chromosome-associated proteins correlate with chronic graft-versus host disease  and, therefore, may provoke a time-dependent transplant rejection in female recipients. However, even with the loss of donor leukocyte function due to the rejection mechanism, there may still be an initial therapeutic window during the first few days or even weeks before the donor leukocytes are rejected. Therefore, the transfer may still be effective in both sex-mismatched settings at least during the first few weeks.
It is also clear that an MHC-mismatch can have a substantial impact on the transferred leukocytes. This finding is not surprising since it is well known that different MHC molecules can provoke strong rejection mechanisms in recipients. However, it is somewhat surprising that donor leukocytes, regardless of whether sex-mismatch or not, faired considerably worse in female recipients than in male recipients in all of the mismatched settings.
Aging also had a profound inhibitory effect on the anticancer activity of SR/CR leukocytes. The effects can be divided into two parts. The first aspect is the age of the donor leukocytes. Our data showed that old donor leukocytes had drastically reduced functionality. It is interesting to note that the age-related loss of function in the leukocytes could not be restored in the young recipients. It appeared that the functional loss in old leukocytes was persistent and structural. However, alternatively the old cells may have simply needed more time in the young environment to regain function before the challenge with the cancer cells. The second aspect is the age of recipients. Our data showed that young leukocytes were inhibited in old recipients. It seems that the young leukocytes were inhibited by either the surface molecules of the host tissues or by circulating diffusible host molecules in body fluids. It has been previously shown that the circulation of old animals contains inhibitory components that affect young tissues and cells . Our observations are consistent with the theme that there are cytotoxic or inhibitory factors in the circulation of old hosts that can abolish the functionality of transplanted young donor leukocytes. One of the intriguing findings was that the deleterious effects of the host environment were not immediate. We observed a period of time during the initial 2-3 weeks, before the subsequent S180 challenges, in which the mice survived the initial S180 challenge similarly to the control. This observation suggests that during the initial 2-3 weeks, the donor leukocytes could function normally before their eventual decline.
Cryopreservation of donor leukocytes is a convenient way of retaining cell viability for a long period of time [13, 14, 16]. However, it was unclear whether the functionality of the stored leukocytes could be maintained. Our studies showed that the anticancer activity of donor leukocytes could be largely maintained during a period of storage and through the processes of freezing and thawing. This observation may open up the possibility of leukocyte banking from young donor leukocytes for either autologous or allogeneic use in the future .
Our results also showed substantial inhibition of donor cell functionality by irradiation. Although the anticancer activity of donor leukocytes does not appear to require additional cell proliferation, the effect of irradiation on donor cell function was considerable. Irradiation at high doses, such as 25 Gy, is known to induce DNA lesions in treated cells . While this treatment can prevent cell proliferation, it can also shut down transcription since intact transcripts may not be able to be generated due to DNA damage. Our unpublished results show that SR/CR leukocytes have a vastly different transcription profile compared to WT leukocytes, and alteration or impairment of gene expression as a result of radiation may cause damage to transcripts required for SR/CR leukocyte killing of cancer cells.
It was encouraging to see that there was a time period during the initial 2-3 weeks in which the recipients' survival was comparable to the controls again suggesting that even in the presence of inhibitory conditions the donor leukocytes were still able to function. The donor leukocytes of the SR/CR mice displayed a meaningful anticancer functionality during this initial time period before their eventual decline. The most likely explanation for this is that the protective and therapeutic functions of SR/CR donor leukocytes against cancer can be accomplished within a few hours or days. This timeframe is well before the host's inhibitory factors have had a chance to abolish leukocyte function. Therefore, it is possible that donor leukocytes may offer a new therapeutic concept in the human setting, in which the recipients would be much older than the donors and the donor-recipient pair would most likely be unavoidably and intentionally mismatched. However, even with these suboptimal conditions, donor leukocytes may still be able to function long enough to provide a beneficial effect.
Cell Lines and Mouse Strains
The S180 cell line was obtained from the American Type Culture Collection (ATCC) (Manassas, VA). S180 cells were propagated in Dulbecco's Modified Eagle Medium (DMEM) with 10% fetal bovine serum (FBS) at 37°C in 5% carbon dioxide, or maintained by serial passage in wild-type (WT) C57BL/6 mice as ascites tumors. C57BL/6 (H-2b) and BALB/c (H-2d) mice were purchased from The Jackson Laboratory and Charles River, respectively. SR/CR mice in C57BL/6 (H-2b) and BALB/c (H-2d) backgrounds were bred at the animal resource program (ARP) facility of Wake Forest University School of Medicine. All mice were housed in plastic cages covered with individual air filter tops, containing corncob bedding, allowed free access to water and chow diet, and exposed to a 12-h fluorescent light/dark cycle. All procedures performed on the mice were in compliance with the guidelines approved by the Institutional Animal Care and Use Committee (IACUC) of Wake Forest University and the National Institutes of Health Guide for the Care and Use of Animals.
For adoptive transfers, donor leukocytes were harvested from the spleens and bone marrow of sacrificed SR/CR mice, pooled in phosphate buffered saline and counted. The various experimental setups used one donor mouse per recipient with the exception of the mismatch experimental setup, which used one donor per recipient for the male donors and approximately 1.5 donors per recipient for the female donors due to differences in the spleen sizes. The number of leukocytes transferred to each donor was well above the minimum number of transferred leukocytes (5 × 106) that are necessary to transfer the SR/CR phenotype (unpublished data). The number of transferred cells was matched within each experimental group based on a cell count or approximate spleen size and ranged from 51 × 106 to 102 × 106 or 1 to 1.5 spleens per recipient. The exception to this range and matching criteria were the cryopreservation adoptive transfer experimental group which had an approximately 2 × 106 difference between the control and treated group (7-9 × 106 for each recipient). The pooled donor leukocytes were then treated, if applicable, and injected intraperitoneally (IP) into recipient mice. One day after the leukocyte transfers, recipient mice were challenged with 1 × 106 S180 to verify resistance. As previously shown , recipient mice survived the S180 challenges only if the transfers of the SR/CR leukocytes were successful.
When desired, leukocytes from donor mice were irradiated in flasks with a total accumulated dose of 25 Gray (Gy) with a cesium irradiator.
For cryopreserving, donor leukocytes were resuspended in a cryopreserving media consisting of 90% FBS +10% Dimethyl Sulfoxide (DMSO) and stored first in a -80°C freezer for approximately one day before being transferred to liquid nitrogen for approximately one month. Before transfers, the stored leukocytes were thawed in a 37°C water bath, washed two times with phosphate buffered saline to remove any remaining fetal bovine serum, and counted via trypan blue exclusion.
At desired time points of the studies, mice were sacrificed. The lungs, livers, intestines, kidneys, spleens and other tissues of potential interest were dissected and stored in 10% neutral buffered formalin. The collected tissues were embedded in paraffin. Sections of the tissues were stained with hematoxylin and eosin and examined for histological abnormalities.
The described studies were supported by grants from the Cancer Research Institute, the National Cancer Institute, and the Charlotte Geyer Foundation (to Z.C.).
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