Researchers designing antitumor treatments have long focused on eliciting tumor-specific CD8 cytotoxic T lymphocytes (CTL) because of their potent killing activity and their ability to reject transplanted organs. The resulting treatments, however, have generally been surprisingly poor at inducing complete tumor rejection, both in experimental models and in the clinic. Although a few scattered studies suggested that CD4 T "helper" cells might also serve as antitumor effectors, they have generally been studied mostly for their ability to enhance the activity of CTL. In this mouse study, we compared monoclonal populations of tumor-specific CD4 and CD8 T cells as effectors against several different tumors, and found that CD4 T cells eliminated tumors that were resistant to CD8-mediated rejection, even in cases where the tumors expressed major histocompatibility complex (MHC) class I molecules but not MHC class II. MHC class II expression on host tissues was critical, suggesting that the CD4 T cells act indirectly. Indeed, the CD4 T cells partnered with NK cells to obtain the maximal antitumor effect. These findings suggest that CD4 T cells can be powerful antitumor effector cells that can, in some cases, outperform CD8 T cells, which are the current "gold standard" effector cell IntroductionResearchers designing antitumor vaccines, or treatments involving transfers of activated antitumor cells, have long focused on methods to elicit tumor-specific CD8 CTLs, envisioning that their potent ability to kill tumor targets in vitro and to reject transplants in vivo would translate into equally potent antitumor activity in vivo. Although many of the resulting treatments have indeed been able to elicit CTLs that recognize tumor cells and/or tumor antigens in vitro, complete tumor regression has been achieved in only a minority of patients. [1][2][3][4][5] Animal models have generated similar results. In a few cases, the transfer of monoclonal T cell receptor transgenic (TCR Tg) CD8 T cells was able to clear small tumors, 6 but in most, the TCR Tg CD8 cells were ineffective without the addition of other aids. In short, though CD8 CTL can clear tumors, they most often do not, unless helped by additional treatments. [6][7][8][9][10][11][12] Over the last 25 years, a few studies have shown that CD4 T cells could also clear tumors completely independently of CD8s. [13][14][15][16][17] Nevertheless, CD4 T cells continue to be studied mainly for their role as helpers for CD8 CTL, 11,18,19 and it has even been suggested that tumor-specific CD4 T regulatory cells could act as suppressors of antitumor responses. 20 Thus, their potential as CD8-independent antitumor effectors has gained only a few proponents, [13][14][15][16][17][21][22][23][24] and only a few of the newly designed cancer vaccines incorporate antigens to stimulate CD4 cells, mostly to enhance their helper activity. 25,26 Most studies using adoptive transfer of tumor-specific T cells continue to focus entirely on CD8 cells. 2,3,[27][28][29][30] We decided to do a direc...

In humans, naturally acquired microchimerism has been observed in many tissues and organs. Fetal microchimerism, however, has not been investigated in the human brain. Microchimerism of fetal as well as maternal origin has recently been reported in the mouse brain. In this study, we quantified male DNA in the human female brain as a marker for microchimerism of fetal origin (i.e. acquisition of male DNA by a woman while bearing a male fetus). Targeting the Y-chromosome-specific DYS14 gene, we performed real-time quantitative PCR in autopsied brain from women without clinical or pathologic evidence of neurologic disease (n = 26), or women who had Alzheimer’s disease (n = 33). We report that 63% of the females (37 of 59) tested harbored male microchimerism in the brain. Male microchimerism was present in multiple brain regions. Results also suggested lower prevalence (p = 0.03) and concentration (p = 0.06) of male microchimerism in the brains of women with Alzheimer’s disease than the brains of women without neurologic disease. In conclusion, male microchimerism is frequent and widely distributed in the human female brain.

BackgroundTransplant rejection has been considered to occur primarily because donor antigens are not present during the development of the recipient's immune system to induce tolerance. Thus, transplantation prior to recipient immune system development (pre-immunocompetence transplants) should induce natural tolerance to the donor. Surprisingly, tolerance was often not the outcome in such 'natural tolerance models'. We explored the ability of natural tolerance to prevent immune responses to alloantigens, and the reasons for the disparate outcomes of pre-immunocompetence transplants.ResultsWe found that internal transplants mismatched for a single minor-H antigen and 'healed-in' before immune system development were not ignored but instead induced natural tolerance. In contrast, multiple minor-H or MHC mismatched transplants did not consistently induce natural tolerance unless they carried chimerism generating passenger lymphocytes. To determine whether the systemic nature of passenger lymphocytes was required for their tolerizing capacity, we generated a model of localized vs. systemic donor lymphocytes. We identified the peritoneal cavity as a site that protects allogeneic lymphocytes from killing by NK cells, and found that systemic chimerism, but not chimerism restricted to the peritoneum, was capable of generating natural tolerance.ConclusionThese data provide an explanation for the variable results with pre-immunocompetence transplants and suggest that natural tolerance to transplants is governed by the systemic vs. localized nature of donor antigen, the site of transplantation, and the antigenic disparity. Furthermore, in the absence of systemic lymphocyte chimerism the capacity to establish natural tolerance to allogeneic tissue appears strikingly limited.ReviewersThis article was reviewed by Matthias von Herrath, Irun Cohen, and Wei-Ping Min (nominated by David Scott).

Stable mixed chimerism has been considered the most robust tolerance strategy. However, rejection of solid donor tissues by chimeras has been observed, a state termed split tolerance. Since new non-myeloablative mixed chimerism approaches are being actively pursued, we sought to determine whether they lead to full tolerance or split tolerance and to define the mechanisms involved. Fully mismatched mixed chimeras generated by induction with various lymphocyte-depleting antibodies along with either low-dose irradiation or busulfan and temporary sirolimus, maintained stable mixed chimerism but nevertheless rejected donor skin grafts. Generation of stable mixed chimerism using antibody targeting CD40L, but not depleting antibodies to CD4 and CD8, could prevent split tolerance when skin grafts were given together with donor bone marrow. Minor antigen matching abrogated the ability of effector T cells to reject donor skin grafts. A CFSE killing assay indicated that chimeras were both directly and indirectly tolerant of donor hematopoietic cell antigens, suggesting that minor mismatches triggered a tissue-specific response. Thus, split tolerance due to tissuerestricted polymorphic antigens prevents full tolerance in a number of nonmyeloablative mixed chimerism protocols and a 'tolerizing' agent is required to overcome split tolerance. A model of the requirements for split tolerance is presented.Supporting information for this article is available at http://www.wiley-vch.de/contents/jc_2040/2007/36938_s.pdf IntroductionMuch of the effort to develop donor-specific transplantation tolerance has been focused on inducing peripheral tolerance through costimulation blockade. While initially promising, with more extensive tests of this approach the success has been somewhat limited, particularly when translated to larger animal models and to the clinic [1]. In hindsight this may not be surprising given that tolerance naturally occurs primarily in the thymus and only secondarily in the periphery [2]. In contrast, the approach of generating hematopoietic chimerism via bone marrow transplantation (BMT) takes advantage of the thymic central tolerance mechanisms, and is considered the most robust method of inducing donor-specific tolerance [3][4][5][6]. The chimer- ism approach is limited clinically by the harsh recipient conditioning needed to establish chimerism and the possibility of graft-vs.-host disease [7]. More recently less toxic strategies have been developed that establish mixed allogeneic chimerism, where substantial levels of donor and recipient hematopoietic cells co-exist in the recipient, and have raised hope that robust transplantation tolerance will soon be routine clinically [6]. Mixed chimerism has been considered to induce tolerance to all other donor tissues. If true, mixed chimerism could be a solution for both solid organ transplantation and cellular transplants, such as allogeneic islets used to treat type-1 diabetes. However, studies in full chimeras, where virtually all hematopoietic cells in the recipien...

Hematopoietic chimerism is considered to generate robust allogeneic tolerance; however, tissue rejection by chimeras can occur. This “split tolerance” can result from immunity toward tissue-specific Ags not expressed by hematopoietic cells. Known to occur in chimeric recipients of skin grafts, it has not often been reported for other donor tissues. Because chimerism is viewed as a potential approach to induce islet transplantation tolerance, we generated mixed bone marrow chimerism in the tolerance-resistant NOD mouse and tested for split tolerance. An unusual multilevel split tolerance developed in NOD chimeras, but not chimeric B6 controls. NOD chimeras demonstrated persistent T cell chimerism but rejected other donor hematopoietic cells, including B cells. NOD chimeras also showed partial donor alloreactivity. Furthermore, NOD chimeras were split tolerant to donor skin transplants and even donor islet transplants, unlike control B6 chimeras. Surprisingly, islet rejection was not a result of autoimmunity, since NOD chimeras did not reject syngeneic islets. Split tolerance was linked to non-MHC genes of the NOD genetic background and was manifested recessively in F1 studies. Also, NOD chimeras but not B6 chimeras could generate serum alloantibodies, although at greatly reduced levels compared with nonchimeric controls. Surprisingly, the alloantibody response was sufficiently cross-reactive that chimerism-induced humoral tolerance extended to third-party cells. These data identify split tolerance, generated by a tolerance-resistant genetic background, as an important new limitation to the chimerism approach. In contrast, the possibility of humoral tolerance to multiple donors is potentially beneficial.