Natural antibodies and the host immune responses to xenografts

Airway reconstruction using decellularized aortic xenografts in a dog model

Tracheal reconstruction after extensive resection remains a challenge in thoracic surgery. Aortic allograft has been proposed to be a potential tracheal substitute. However, clinically, its application is limited for the shortage of autologous aortic segment. Whether xenogeneic aortic biosheets can be used as tracheal substitutes remains unknown. In the present study, we investigated the possibility in dog model. The results show that all dogs were survived without airway symptoms at 6 months after tracheal reconstruction with gently decellularized bovine carotid arteries. In the interior of engrafted areas, grafted patch integrated tightly with the residual native tracheal tissues and tracheal defects in the lumen were repaired smoothly without obvious inflammation, granulation, anastomotic leakage, or stenosis. In addition, histological and scanning electron microscopy examination showed that grafted patches were covered with ciliated columnar epithelium similar to epithelium in native trachea, which indicated successfully re-epithelialization of decellularized bovine carotid arteries in dogs. These findings provide preclinical investigation of xenogeneic aortic biosheets in serving as tracheal substitute in a dog model, which proposes that decellularized biosheets of bovine carotid may be a potential material for bioartificial tracheal graft.

Small Diameter Xenogeneic Extracellular Matrix Scaffolds for Vascular Applications

Currently, despite the success of percutaneous coronary intervention (PCI), coronary artery bypass graft (CABG) remains among the most commonly performed cardiac surgical procedures in the United States. Unfortunately, the use of autologous grafts in CABG presents a major clinical challenge as complications due to autologous vessel harvest and limited vessel availability pose a significant setback in the success rate of CABG surgeries. Acellular extracellular matrix (ECM) scaffolds derived from xenogeneic vascular tissues have the potential to overcome these challenges, as they offer unlimited availability and sufficient length to serve as "off-the-shelf" CABGs. Unfortunately, regardless of numerous efforts to produce a fully functional small diameter xenogeneic ECM scaffold, the combination of factors required to overcome all failure mechanisms in a single graft remains elusive. This article covers the major failure mechanisms of current xenogeneic small diameter vessel ECM scaffolds, and reviews the recent advances in the field to overcome these failure mechanisms and ultimately develop a small diameter ECM xenogeneic scaffold for CABG. Impact Statement Currently, the use of autologous vessel in coronary artery bypass graft (CABG) is common practice. However, the use of autologous tissue poses significant complications due to tissue harvest and limited availability. Developing an alternative vessel for use in CABG can potentially increase the success rate of CABG surgery by eliminating complications related to the use of autologous vessel. However, this development has been hindered by an array of failure mechanisms that currently have not been overcome. This article describes the currently identified failure mechanisms of small diameter vascular xenogeneic extracellular matrix scaffolds and reviews current research targeted to overcoming these failure mechanisms toward ensuring long-term graft patency.

Repertoire of Abs primed by bacteria in gnotobiotic mice

Innate immunity natural Abs (NAbs) execute a number of functions, including protection and surveillance. Despite active research, the stimuli that induce the formation of NAbs are still described only hypothetically. Here, we compared repertoires of anti-glycan Abs in the peripheral blood of mice that received per os various bacteria. The repertoires of Abs of mice primed in this way were compared using a microarray that included about 350 glycans, as well as 150 bacterial polysaccharides. Sterile mice did not possess anti-glycan Abs. Oral inoculation of a single strain or combination of two to four strains of bacteria, as well as putting the animals on short-term nutrition with non-sterile food, did not contribute significantly to the formation of Abs, whereas a single gavage of digested food of non-sterile mice induced the formation of a repertoire close to the natural ones. Interestingly, the priming with polysaccharide Ags (in a composition of the bacterial cell envelope), that is, dominant Ags of bacteria, led to the induction of Abs against typical glycans of mammalian glycoproteins and glycolipids (e.g. Abs of the ABH blood group system) that do not have a structural similarity to the polysaccharides. The results support the importance of early contact with a naïve immune system with microorganisms of the environment to form a normal NAbs repertoire.

Increase in anti-Gal IgM level is associated with early graft failure in intraportal porcine islet xenotransplantation

BACKGROUND

Anti-Gal is a major antibody induced in non-human primates (NHPs) after xenotransplantation. To understand the mechanism of graft rejection, we investigated the association between anti-Gal responses and graft failure in NHP recipients of porcine islet transplantation (PITx).

METHODS

Intraportal PITx was performed in 35 diabetic NHPs, and graft function was monitored. Early graft failure (EGF) was defined as loss of graft function within a month after PITx. Seven, 19, nine NHPs received immunosuppression (IS) without CD40 pathway blockade (Group I), with anti-CD154 (Group II), and with anti-CD40 (Group III), respectively. The anti-Gal levels on day 0 and day 7 of PITx were measured by ELISA.

RESULTS

The frequency of EGF was significantly lower in Group II (26.3%) than in Group I (100%, P=0.0012) and Group III (77.8%, P=0.0166). While levels of anti-Gal IgG in Group I and anti-Gal IgM in Group III increased on day 7 compared with day 0 (P=0.0156 and 0.0273), there was no increase in either on day 7 in Group II. The ratio of anti-Gal IgM or IgG level on day 7 to that on day 0 (Ratio7/0) was significantly higher in recipients with EGF than without EGF (P=0.0009 and 0.0027). ROC curve analysis of anti-Gal IgM Ratio7/0 revealed an area under the curve of 0.789 (P=0.0003).

CONCLUSIONS

IS with anti-CD154 suppressed anti-Gal responses and prevented EGF in PITx. Anti-Gal IgM Ratio7/0, being associated with EGF, is a predictive marker for EGF.

Human platelet lysate supplementation of mesenchymal stromal cell delivery: issues of xenogenicity and species variability

Immunogenicity of fetal bovine serum (FBS) poses a problem for its use in the propagation of autologous mesenchymal stromal cells (MSCs) for cell therapy. Human platelet lysate (hPL), an enriched growth factor solution containing mitogenic and angiogenic cues, has potential utility in replacing FBS for human MSC (hMSC) delivery strategies. Despite its potentiation of hMSC number in vitro, little is known concerning its capacity to supplement implanted hMSC-seeded constructs and promote tissue regeneration in vivo. In this study, we tested the effects of incorporating hPL in cell-seeded constructs implanted subcutaneously into immunocompromised rats, investigated in vitro interactions between hPL and rat MSCs (rMSCs) and determined interspecies variability in the PL product [hPL vs rat PL (rPL)] and its effect on cultured MSCs (hPL/hMSCs vs rPL/rMSCs). The overarching aim was to determine the utility of hPL to foster MSC survival in preclinical rodent models. Exposure to hPL-supplemented media resulted in rMSC death, by a process attributable to heat-labile proteins, but not membrane attack complex formation. In the in vitro syngeneic model, the rodent product proved fundamentally distinct from the human product, with rPL having substantially lower growth factor content than hPL. Moreover, contrary to the positive effects of hPL on hMSC expansion, rPL did not reduce rMSC doubling time for the serum concentrations examined. When tested in vivo, hPL did not improve cell survival within hydrogel constructs through 2 weeks postimplantation. In summary, this study highlights the many facets of xenogenicity and interspecies variability that must be considered in the preclinical evaluation of hPL. Copyright © 2016 John Wiley & Sons, Ltd.

Progress towards inducing tolerance of pig-to-primate xenografts

Xenotransplantation remains the best near-term solution to the shortage of transplantable organs that currently limits the field of transplantation. However, because the immune response to xenografts is considerably stronger than it is to allografts, the amount of non-specific immunosuppression required to avoid xenograft rejection may limit clinical applicability. For this reason, we consider it likely that the success of clinical xenotransplantation will depend on finding ways of safely inducing tolerance across xenogeneic barriers rather than relying entirely on non-specific immunosuppressive agents. In this laboratory, two approaches are being studied for the induction of pig-to-primate tolerance: a) the simultaneous transplantation of vascularized thymus and solid organs; and b) mixed hematopoietic chimerism. A summary of the development of these two approaches and their current status is the subject of this review.

PET Imaging of Serotonin Transporters With 4-[(18)F]-ADAM in a Parkinsonian Rat Model With Porcine Neural Xenografts

Parkinson's disease (PD) is a neurodegenerative disease characterized by a loss of dopaminergic neurons in the nigrostriatal pathway. Apart from effective strategies to halt the underlying neuronal degeneration, cell replacement now offers novel prospects for PD therapy. Porcine embryonic neural tissue has been considered an alternative source to human fetal grafts in neurodegenerative disorders because its use avoids major practical and ethical issues. This study was undertaken to evaluate the effects of embryonic day 27 (E27) porcine mesencephalic tissue transplantation in a PD rat model using animal positron emission tomography (PET) coupled with 4-[(18)F]-ADAM, a serotonin transporter (SERT) imaging agent. The parkinsonian rat was induced by injecting 6-hydroxydopamine into the medial forebrain bundle (MFB) of the right nigrostriatal pathway. The apomorphine-induced rotation behavioral test and 4-[(18)F]-ADAM/animal PET scanning were carried out following 6-OHDA lesioning. At the second week following 6-OHDA lesioning, the parkinsonian rat rotates substantially on apomorphine-induced contralateral turning. In addition, the mean striatal-specific uptake ratio (SUR) of 4-[(18)F]-ADAM decreased by 44%. After transplantation, the number of drug-induced rotations decreased markedly, and the mean SUR of 4-[(18)F]-ADAM and the level of SERT immunoreactivity (SERT-ir) in striatum were partially restored. The mean SUR level was restored to 71% compared to that for the contralateral intact side, which together with the abundant survival of tyrosine hydroxylase (TH) neurons accounted for functional recovery at the fourth week postgraft. In regard to the extent of donor-derived cells, we found the neurons of the xenografts from E27 transgenic pigs harboring red fluorescent protein (RFP) localized with TH-ir cells and SERT-ir in the grafted area. Thus, transplanted E27 porcine mesencephalic tissue may restore dopaminergic and serotonergic systems in the parkinsonian rat. The 4-[(18)F]-ADAM/animal PET can be used to detect serotonergic neuron loss in PD and monitor the efficacy of therapy.

Divergence of the response induced by xenogenic immunization in the sepsis survival of rats

We have previously described that boosted natural xenoantibodies in rats cross-react to bacteria by targeting carbohydrate antigens. This type of immunization is associated with reduced survival after cecal ligation and puncture (CLP). In the present study, we investigated further this phenomenon by immunizing Lewis rats with three intraperitoneal injections, every other day, of hamster blood compared to saline-injected control animals. One day after the last injection, CLP was performed to produce a low-grade sepsis. Induction of xenoantibodies was associated with a reduction in animal survival after CLP relative to controls (45% vs. 90%, p<0.01). No bacterial blood load was observed after CLP in this model either with or without xenoantibody enhancement, indicating that the augmented mortality was not mediated by a direct effect of boosted xenoantibodies over blood bacteria. Nevertheless, the xenoimmunization produced a systemic inflammatory response in all rats. Additionally, a lack of weight gain at the time of CLP was present in animals that died after the procedure, which was not observed in surviving rats and controls. The cytokine profile at the time of CLP in animals that died after the procedure was characterized by an increase in the serum level of several cytokines, particularly adipokines. In contrast, the cytokine profile at CLP of xenoimmunized rats that survived the procedure was characterized by a reduction in the level of cytokines. In conclusion, this study failed to show a direct effect of boosted xenoantibodies over blood bacterial isolates as cause for the decreased survival after CLP. However, it evidenced that non-infectious systemic inflammation may lead to a pattern of augmented cytokines, particularly adipokines, which impairs survival after subsequent CLP. Therefore, the profile of cytokines existing before the infectious insult appears more crucial than that resulting from the condition for the outcome of sepsis.

Anti-Gal IgM, IgA and IgG natural antibodies in childhood

The target for the most abundant xenoreactive natural antibodies in humans is the α-Gal epitope. Anti-Gal could provide natural immune defense against pathogens that express the α-Gal epitope. Anti-Gal natural antibodies are usually studied in adult individuals. Data demonstrating the incidence and concentration of anti-Gal natural antibodies in childhood are in short supply and incomplete. In the present study we prospectively quantified anti-Gal IgM, IgA and IgG levels in different age groups of children from delivery to 24 months of age and compared these levels to the level of these antibodies in their respective mothers. Measurement of anti-Gal antibodies may broaden the spectrum of specific antibodies that are available for determination of specific antibody responses in physiological and pathological conditions in children. Plasma was collected from umbilical cord blood of full term newborn, from blood of infants at age 6, 12 and 24 months and from their respective mothers at time of delivery. Quantitative determination of anti-Gal antibodies IgM, IgA and IgG were made with the enzyme immunoassays Human Anti-Alpha Galactosyl IgM ELISA, IgG ELISA and IgA ELISA. Hemagglutination activity was titrated against rabbit erythrocytes. The kinetic processes for the formation of natural antibodies in the first two years of life, in general, compared with the kinetics for the formation of total immunoglobulins IgM, IgA and IgG. There were no detectable anti-Gal IgM and IgA in the cord blood, whereas anti-Gal IgG were found at similar levels in both neonate cord blood and peripheral blood of their respective mothers. When comparing the percentage of natural antibodies in the plasma of children, the level of natural antibodies in children at the age of two years was approximately 37% for IgM, 25% for IgG and 15% for IgA. The titration of antibodies required for agglutination of rabbit red blood cells over the 24 month period followed the same trend observed for the formation of natural antibodies. Our study demonstrates the kinetics of formation of anti-Gal IgM, IgA and IgG natural antibodies in the first two years of life. The relative lack of these antibodies in this period should be taken into account when assessing for humoral immunodeficiencies, particularly with regards to the potential for children to mount an anti-carbohydrate response.

Boosted rat natural xenoantibodies cross-react with Enterococcus faecalis by targeting melibiose and L-rhamnose

Natural antibodies include a subset described as xenoantibodies considered to be directed at microorganisms and also cross-react with antigens of unrelated species. In this study, we generated T-cell-independent (TI) and T-cell-dependent (TD) xenoantibodies in Lewis rats with hamster and pig blood injections. TI anti-hamster and anti-pig IgM and IgG xenoantibodies cross-reacted with Enterococcus faecalis but not with Escherichia coli isolated from the blood of Lewis rats after cecal ligation and puncture (CLP). TI anti-pig IgM xenoantibodies also showed some reactivity with two human blood isolates of E. faecalis. In contrast, TD xenoantibodies did not show any reactivity with rat or human bacteria. TI and TD anti-hamster and anti-pig IgM and IgG xenoantibodies showed cross-reactivity with lymphocytes and endothelial cells from species distinct to that used for immunization. Glycan array analysis and inhibition assays identified antibodies against melibiose and L-rhamnose as mediators of anti-hamster and anti-porcine xenoantibody cross-reactivity with E. faecalis. A rise in TI anti-hamster and anti-pig xenoantibodies was accompanied by decreased survival of Lewis rats in a low-severity sepsis model of CLP. Therefore, TI xenoantibodies in the rat include anti-carbohydrate antibodies reactive to bacteria of endogenous flora. Enhancement of these antibodies may result in more severe infectious diseases caused by these microorganisms.

Natural IgG antibody with anti-β-galactosyl specificity suppressed hepatoma cell invasion in culture

The effect of natural IgG antibody recognizing β-galactosyl epitope on hepatoma cell invasion was investigated. Anti-β-galactosyl antibody dose-dependently suppressed hepatoma invasion underneath primarily cultured mesothelial cells monolayer without affecting the proliferation, to the same extent as natural IgG antibody with anti-α-galactosyl specificity, which had already been reported to have an anti-metastatic activity. The inhibitory effect of anti-β-galactosyl antibody was completely canceled by adding lactose (galactose-β-1, 4-glucose) to the medium, indicating that this antibody recognized some antigens with β-galactosyl epitope. Hepatoma cells pretreated with this antibody for 48 h showed reduced invasive activity, while the pretreatment of mesothelial cells with the antibody did not affect hepatoma cells invasion. Anti-β-galactosyl antibody also suppressed hepatoma cells adhesion to mesothelial cells monolayer. These results suggest that natural antibody with anti-β-galactosyl specificity may recognize the β-galactosyl epitope in some adhesion-related molecules on hepatoma cells, thus suppressing adhesion and invasion to mesothelial cells monolayer. These results suggest possible therapeutic uses of this antibody in the treatment of metastatic tumors.

Polyreactive antibodies in multidonor-derived immunoglobulin G: theory and conclusions drawn from experiments

Multidonor-derived (md) preparations of IgG antibodies, agents of therapeutic potential, contain molecules interacting at clonal concentrations (concns) and with affinities recently estimated to cover a considerable range. Here we demonstrate that polyreactivity of the monomeric molecules represents the essential driving force of formation of the main reaction product, the IgG-dimers. This conclusion is obtained by applying the principles of the law of mass action to dimer formation by polyreactive monomeric reactants. In addition, general interrelationships involving the mean number of reactants per reactor, the experimental dimer portion (w/w) and the mean concentrations of monomers in a polyreactive and monoreactive antibody system are derived. These interrelationships, together with quantitative results obtained from simplified computational kinetic models of polyreactive antibodies, allow to estimate a remarkably high value for the mean number of reactants per reactor, exceeding 60 for the underlying IgG preparation obtained from pooled human plasma units of 5000 donors. Moreover, the potential origin and other consequences of polyreactivity are outlined.

B-1 cell participation in T-cell-mediated alloimmune response

B-1 and B cells are important producers of natural antibodies in mice and humans and, therefore, are considered as the first line of defense against pathogens. Because of that, their role in T-cell-mediated immune responses is commonly underrated. However, recent studies have described the participation of B-1 cells in immediate and delayed-type hypersensitivity. The present work assessed the role of B-1 cells in the rejection of allografts in mice, an immune reaction mainly orchestrated by T cells. We have transplanted allogeneic skin and heart to wild-type and B-1-cell-deficient mice, and followed rejection kinetics. Skin graft-infiltrating cells were analyzed by flow cytometry. We observed a delay in rejection kinetics of B-1-cell-deficient mice when compared to wild-type mice. Adoptive transfer of B-1 cells into B-1-cell-deficient mice abrogated this delay. The longer survival observed in the absence of B-1 cells correlated with less CD8+ T cells infiltrating the grafts, as well as with more mast cells. Collectively, our results show the participation of B-1 cells in the allograft rejection process in mice and collaborate to the understanding of B-1 cell biology.

Autologous fibrin-coated small-caliber vascular prostheses improve antithrombogenicity by reducing immunologic response

OBJECTIVE

We have recently developed a thrombin-free fibrin-coated vascular prosthesis that has a high performance rate in producing graft antithrombogenicity. We hypothesized that autologous, compared with xenologous, fibrin coatings could improve the antithrombogenicity of grafts by reducing immunologic response.

METHODS

Autologous fibrin-coated vascular prostheses and/or xenologous fibrin-coated vascular prostheses (internal diameter, 2 mm; length, 2.5 cm) were implanted in the bilateral carotid arteries of 50 Japanese white rabbits. They were classified into 2 groups by the selection of grafts in the individual: group I (autologous fibrin-coated vascular prosthesis and xenologous fibrin-coated vascular prosthesis); and group II (group IIa: both autologous fibrin-coated vascular prostheses, or group IIx: both xenologous fibrin-coated vascular prostheses). During a maximum of 180 days after implantation, we evaluated the thrombotic, inflammatory, and immunologic responses associated with both types of graft.

RESULTS

All grafts were patent at each end point. In group I, both platelet deposition and anti-graft antibodies in autologous fibrin-coated vascular prostheses were significantly less than those in xenologous fibrin-coated vascular prostheses until postoperative day 30. At postoperative day 10, there were significantly fewer CD45-positive infiltrating cells in autologous fibrin-coated vascular prostheses, and intercellular adhesion molecule-1, vascular cell adhesion molecule-1, and nuclear factor-kappa B expression in autologous fibrin-coated vascular prostheses were less than those in xenologous fibrin-coated vascular prostheses. The neointimal hyperplasia in autologous fibrin-coated vascular prostheses was significantly decreased at postoperative day 180. In group II, serial changes of serum levels of immunoglobulin M, immunoglobulin G, interleukin-1beta, and tissue-type plasminogen activator/plasminogen activator inhibitor-1 ratio in autologous fibrin-coated vascular prostheses were significantly less than those in xenologous fibrin-coated vascular prostheses. In both grafts, platelet deposition significantly correlated with serum immunoglobulin G level and tissue-type plasminogen activator/plasminogen activator inhibitor-1 ratio.

CONCLUSION

These findings suggest that autologous fibrin coating in thrombin-free fibrin-coated vascular prostheses improve antithrombogenicity by reducing immunologic response and have a potential for clinical use in hybrid small-caliber vascular grafts.

Identification of the V genes encoding xenoantibodies in non-immunosuppressed rhesus monkeys

The major immunological barrier that prevents the use of wild-type pig xenografts as an alternative source of organs for human xenotransplantation is antibody-mediated rejection. In this study, we identify the immunoglobulin variable region heavy (IgV(H)) chain genes encoding xenoantibodies to porcine heart and fetal porcine islet xenografts in non-immunosuppressed rhesus monkeys. We sought to compare the IgV(H) genes encoding xenoantibodies to porcine islets and solid organ xenografts. The immunoglobulin M (IgM) and IgG xenoantibody response was analysed by enzyme-linked immunosorbent assay and cDNA libraries from peripheral blood lymphocytes were prepared and sequenced. The relative frequency of IgV(H) gene usage was established by colony filter hybridization. Induced xenoantibodies were encoded by the IGHV3-11 germline progenitor, the same germline gene that encodes xenoantibodies in humans mounting active xenoantibody responses. The immune response to pig xenografts presented as solid organs or isolated cells is mediated by identical IgV(H) genes in rhesus monkeys. These animals represent a clinically relevant model to identify the immunological basis of pig-to-human xenograft rejection.

Xenotransplantation: an update on recent progress and future perspectives

Currently, the number of patients awaiting transplantation is continuously increasing, and shortage of available deceased organ donors is the major limitation for organ and cell allotransplantation. Research to develop alternative sources of tissues is ongoing and xenogeneic organs or cells represent an attractive solution. This review focuses on recent progress achieved in this field, including the development of newly genetically modified animal donors and new immunosuppressive approaches. As xenotransplantation is moving closer to clinical application, future perspectives must establish guidelines to ensure that future clinical trials are carried out ethically and safely.

The in vitro and in vivo effects of anti-galactose antibodies on endothelial cell activation and xenograft rejection

We have previously produced a series of antigalactose (anti-Gal) hybridomas and characterized their heavy chain gene usage. Here we have quantified the affinity of these Abs for the alpha-Gal epitope and characterized their in vitro effects on endothelial cell activation and apoptosis. We report that anti-Gal mAbs derived from Gal(-/-) mice show a range of affinity for the alpha-Gal epitope, and that affinity was generally increased as the V(H) gene usage transitioned from germline sequences to sequences exhibiting somatic maturation. Despite an 85-fold range in affinity, all the anti-Gal mAbs examined induced alpha-Gal-specific endothelial cell activation, and after prolonged exposure induced endothelial cell apoptosis in a complement-independent manner. Only murine anti-Gal mAbs of the IgM or IgG3 subclass, but not IgG1, were effective at initiating complement-dependent cell lysis. Using a novel rat to mouse xenograft model, we examined the in vivo ability of these mAbs to induce xenograft rejection and characterized the rejection using histology and immunohistochemistry. Infusion of complement-fixing IgG3 mAbs resulted in either hyperacute rejection or acute vascular rejection of the xenograft. Surprisingly, infusion of an equal amount of a high affinity anti-Gal IgG1 mAb, that fixed complement poorly also induced a rapid xenograft rejection, which we have labeled very acute rejection. These studies emphasize the importance of in vivo assays, in addition to in vitro assays, in understanding the role of anti-Gal IgG-mediated tissue injury and xenograft rejection.

Functional and morphological development of lymphoid tissues and immune regulatory and effector function in rhesus monkeys: cytokine-secreting cells, immunoglobulin-secreting cells, and CD5(+) B-1 cells appear early in fetal development

Little is known regarding the timing of immune ontogeny and effector function in fetal humans and nonhuman primates. We studied the organization of lymphocyte and antigen-presenting cell populations in developing lymphoid tissues of rhesus monkey fetuses during the second and third trimesters (65 to 145 days of gestation; term = 165 days). Immunoglobulin-secreting and cytokine-secreting cells were detected at day 80. The thymus, spleen, lymph nodes, and intestinal mucosa were examined for cells expressing CD3, CD5, CD20, CD68, p55, and HLA-DR. In the spleens of 65-day-old fetuses (early second trimester), the overwhelming majority of total lymphocytes were CD5(+) CD20(+) B-1 cells. The remaining lymphocytes were CD3(+) T cells. By day 80, splenic B and T cells were equal in number. Intraepithelial CD3(+) CD5(-) T cells and lamina propria CD20(+) CD5(+) B cells were present in the intestines of 65-day-old fetuses. By day 80, numerous CD20(+) CD5(+) B cells were present in the jejunums and colons and early lymphocyte aggregate formation was evident. The spleens of 80- to 145-day-old fetuses contained immunoglobulin M (IgM)-secreting cells, while IgA-, IgG-, interleukin-6-, and gamma interferon-secreting cells were numerous in the spleens and colons. Thus, by the second trimester, the lymphoid tissues of the rhesus monkey fetus have a complete repertoire of properly organized antigen-presenting cells, T cells, and B cells.