The characteristics, isolation and synthesis of the phagocytosis stimulating peptide tuftsin

In vitro biological evaluation of new antimycobacterial salicylanilide-tuftsin conjugates

Tuberculosis is caused by Mycobacterium tuberculosis, an intracellular pathogen that can survive in host cells, mainly in macrophages. An increase of multidrug-resistant tuberculosis qualifies this infectious disease as a major public health problem worldwide. The cellular uptake of the antimycobacterial agents by infected host cells is limited. Our approach is to enhance the cellular uptake of the antituberculars by target cell-directed delivery using drug-peptide conjugates to achieve an increased intracellular efficacy. In this study, salicylanilide derivatives (2-hydroxy-N-phenylbenzamides) with remarkable antimycobacterial activity were conjugated to macrophage receptor specific tuftsin based peptide carriers through oxime bond directly or by insertion of a GFLG tetrapeptide spacer. We have found that the in vitro antimycobacterial activity of the salicylanilides against M. tuberculosis H₃₇Rv is preserved in the conjugates. While the free drug was ineffective on infected macrophage model, the conjugates were active against the intracellular bacteria. The fluorescently labelled peptide carriers that were modified with different fatty acid side chains showed outstanding cellular uptake rate to the macrophage model cells. The conjugation of the salicylanilides to tuftsin based carriers reduced or abolished the in vitro cytostatic activity of the free drugs with the exception of the palmitoylated conjugates. The conjugates degraded in the presence of rat liver lysosomal homogenate leading to the formation of an oxime bond-linked salicylanilide-amino acid fragment as the smallest active metabolite.

Drug delivery strategies for therapy of visceral leishmaniasis

IMPORTANCE OF THE FIELD

Visceral leishmaniasis (VL) is the most overwhelming type of leishmaniasis associated with the poverty of developing countries and usually mortal if untreated. Most of the conventionally used dosage forms offer us the shortcomings of toxic side effects and emergence of drug resistance. Several efforts have been made to overcome the barriers involved in the treatment of VL. Colloidal carriers extensively represent the drug delivery systems (DDSs) for intracellular localization of antileishmanial compounds in macrophage-rich organs such as liver, spleen and bone marrow. These DDSs offer superior therapeutic efficacy over the conventional treatment in terms of site-specific drug delivery with reduced side effects. However, after 35 years of research in the field, AmBisome (Amphotericin B liposome for injection, Astellas Pharma US, Inc.) is the only DDS used against the VL.

AREAS COVERED IN THIS REVIEW

A literature search was performed (for drugs and DDSs against VL) on PubMed and through Google.

WHAT THE READER WILL GAIN

This review aims to describe the pathophysiology of VL and its current conventional treatment with special reference to DDSs designed against VL.

TAKE HOME MESSAGE

On reviewing the conventional drugs and DDSs developed against VL, it is concluded that advances in the field of targeted drug delivery can result in more efficient strategies for the therapy of VL.

Synthesis of tritiated tuftsin and macrophage inhibitory tripeptide via acetylenic intermediates

Acetylenic analogues of tuftsin (Thr-Dah-Pro-Arg) and of a macrophage inhibitory tripeptide (Thr-Dah-Pro) have been synthesized by conventional procedures in solution (Dah = 2,6-diamino-4-hexynoic acid). These acetylenic derivatives are intermediates for the preparation of structurally unmodified, tritiated peptides. Catalytic tritiation of Thr-Dah-Pro-Arg and of Thr-Dah-Pro has afforded the radioactive tetra- and tripeptides with specific activities of 11.4 Ci/mmol and 37 Ci/mmol, respectively.

The use of synthetic peptides in the delineation of immunoglobulin antigenic epitopes and Fc effector functions

As an alternative strategy to the use of proteolytic and chemical cleavage in the production of fragments of immunoglobulins retaining Fc effector functions, peptides representative of amino acid sequences constituting the putative active sites have been synthesized and assessed for biological activity in various in vitro systems. This approach has been adopted in attempts to define more precisely the autoantigenic epitope on human IgG against which anti-gamma-globulin antibodies (the so-called general 'rheumatoid factors'), found in the sera and joint fluids of patients with rheumatoid arthritis, are directed. Synthetic peptides representative of epsilon-chain sequences are being used in the production of antibodies (polyclonal and monoclonal) directed against specific epitopes within the Fc regions of human and rat IgE. The ability of these antisera to influence the in vitro functional properties of IgE anaphylactic antibodies is now under investigation, with particular attention being focused on cytophilicity and mast cell triggering. Preliminary findings suggest that certain of the antisera might be capable of inhibiting mast cell sensitization by IgE antibodies, and therefore might form the basis of a new type of anti-allergy compound.

The peptide molecular links between the central nervous and the immune systems

The central nervous system (CNS) and the immune system were for many years considered as two autonomous systems. Now, the reciprocal connections between them are generally recognized and very well documented. The links are realized mainly by various immuno- and neuropeptides. In the review the influence of the following immunopeptides on CNS is presented: tuftsin, thymulin, thymopoietin and thymopentin, thymosins, and thymic humoral factor. On the other side, the activity in the immune system of such neuropeptides as substance P, neurotensin, some neurokinins, enkephalins, and endorphins is discussed.

Autotransplant of spleen tissue in children with schistosomiasis: evaluation of splenic function after splenosis

Autotransplantation of spleen tissue has been done, in the past ten years, in children with schistosomiasis mansoni with bleeding varices. The purposes of this investigation were: (1) to study the morphology and function of the remnant spleen tissue; (2) to quantify the production of tuftsin; and (3) to assess the immune response to pneumococcal vaccine of these patients. Twenty three children, who underwent splenectomy and autologous implantation of spleen tissue into the greater omentum were included in this investigation. The average postoperative follow-up is five years. Splenosis was proved by colloid liver-spleen scans. Search for Howell-Jolly bodies assessed the filtration function. Tuftsin and the titer of pneumococcal antibodies were quantified by ELISA. Splenosis was evident in all children; however, it was insufficient in two. Howell-Jolly bodies were found only in these two patients. The mean tuftsin serum concentration (335.0 +/- 29.8 ng/ml) was inside the normal range. The immune response to pneumococcal vaccination was adequate in 15 patients; intermediate in four; and inadequate in four. From the results the following conclusions can be drawn: splenosis was efficient in maintaining the filtration splenic function in more than 90% and produced tuftsin inside the range of normality. It also provided the immunologic splenic response to pneumococcal vaccination in 65% of the patients of this series.

Tuftsin: on the 30-year anniversary of Victor Najjar's discovery

After a short description of the results of Victor Najjar's research on tuftsin and of the discoveries done by other authors in the early stage of tuftsin investigation, the current state of work on tuftsin is presented, based mainly on the literature published in the years 1984-1997. The presentation follows this order: the occurrence of tuftsin and retro-tuftsin sequences in proteins, their synthesis and biology, the antigenic properties of tuftsin, its influence on phagocytic cells, and other biologic activities of tuftsin, including antimicrobial, antiviral, antitumor and central effects, and the search for tuftsin superactive analogs.

Polytuftsin: its possible effects and mechanism during macrophage activation

Polytuftsin (PT) a 35-40 repeat unit of tuftsin (TKPR), when administered as a conjugate with the malarial peptide, ring-infected erythrocyte surface antigen (RESA), enhanced antigen-induced lymphoproliferation and antibody levels in mice as compared to RESA alone. This enhancement was unrelated to the H-2 background of the animals. The present study was undertaken with a view to understanding the mechanism(s) responsible for this immune enhancement. Peritoneal adherent cells (PAC) from H-2b and H-2d mice were incubated with RESA alone, PT-conjugated RESA, a physical mixture of RESA + PT and PT alone. They were subsequently evaluated for I-A expression using monoclonal antibodies and flow cytometry as well as cell-ELISA. Significant increase in I-A expression on PAC was observed in all 4 groups as compared to untreated cells. Whereas cells treated with PT-conjugated RESA showed highly significant increase in I-A (P < 0.001), the other groups showed moderate increase (P < 0.05). This enhancement was attributable to increase in the number of I-A-positive cells rather than I-A molecules per cell. Moreover, IL-1 release, as assayed by bioassay, was significantly higher in cells treated with conjugated RESA as compared to cells treated with RESA or PT alone (P < 0.05). Thus, it would appear that PT-conjugated RESA peptide of the malarial antigen selectively enhances major histocompatibility complex (MHC) class II molecules on antigen-presenting cells (APC) and may therefore improve immune functions by stimulating better antigen presentation and proliferation of T cells.

Actions of selected proteins, peptides and amino acid derivatives on mouse embryonic development in vitro

1. The actions of various compounds with antiproliferative and/or immunomodulatory activities including the ribosome-inactivating protein gelonin, the tetrapeptide tuftsin, the opioid peptide leucine-enkephalin, the antireproductive tripeptide Thr-Ser-Lys, the melatonin analog 6-methoxy-2-benzoxazolinone, and the amino acid derivatives 5-hydroxytryptamine and oxalysine on mouse embryonic development and organogenesis in vitro were investigated. 2. The various compounds were tested up to a concentration of 300 micrograms/ml. It was found that only 6-methoxy-2-benzoxazolinone, a derivative of tryptophan, produced an increase in the number of abnormal embryos and reductions in the final somite numbers and axial lengths of embryos. 3. 6-Methoxy-2-benzoxazolinone treatment also resulted in an increased incidence of abnormal morphogenesis of various organ primordia, including abnormal yolk sac circulation, twisting of body axis, absence of forelimb bud and opening of cranial neural tube.

Splenic autotransplantation after splenectomy: tuftsin activity correlates with residual splenic function

This study set out to determine the relationship between splenic function (as assessed by the percentage of pitted red cells) and tuftsin activity, and to confirm the return of effective splenic function after splenectomy for trauma. Twenty-three patients (13 men) took part. Ten of mean age 48.5 (range 30-74) years had had the spleen removed for traumatic rupture and 13 of mean age 49.7 (range 23-66) years had undergone elective splenectomy. At the time of the study all patients had had the spleen removed a minimum of 1 year previously (mean 6.1 (range 1-15) years). Fifty healthy volunteers matched for sex and age were also studied. In each subject, residual splenic function was evaluated by counting the percentage of pitted red cells. Tuftsin activity was also determined. A highly significant negative correlation was found between pitted red cell percentage and tuftsin activity (rs = -0.80, P < 0.001). Compared with healthy controls (mean 21.6 (range 13-37) per cent), tuftsin activity was significantly reduced both in patients who had undergone splenectomy for trauma (mean 4.4 (range 0-9) per cent; P < 0.0001) and in those who had had elective splenectomy (mean 0; P < 0.0001). Tuftsin activity was significantly (P < 0.001) more depressed after elective than traumatic splenectomy. These data confirm a decrease in tuftsin activity following splenectomy and show that this deficit is significantly greater after elective than emergency removal. These observations confirm that residual splenic function is often present after traumatic splenectomy.

Suppression of tuftsin activity by the partial sequences of adenovirus type 2 proteins

The synthesis of tuftsin inhibitor-like partial sequences of adenovirus type 2 proteins: TVPPR (1), TRPPR (2), TGPPT (3), PRPPT (4), FVPPR (5), ARPPA (6) and YGPPK (7) are described. The choice of peptides for synthesis was performed by using a simple computer-aided method for predicting antigenic determinants of proteins. For all these peptides an inhibitory activity against tuftsin was found.

Tuftsin deficiency in AIDS

Tuftsin is an endogenous tetrapeptide that stimulates phagocytosis and is released from the Fc fragment of IgG by a splenic endocarboxypeptidase. Tuftsin activity and splenic function were measured in 21 patients with AIDS, 7 patients with AIDS-related complex (ARC), 22 patients who had undergone splenectomy, and 37 healthy volunteers. There was a significant inverse correlation between tuftsin activity and splenic function in all subjects. Tuftsin activity was significantly lower in patients with AIDS, ARC, and in those who had undergone splenectomy compared with healthy volunteers. Tuftsin deficiency may contribute to the risk of bacterial infection in symptomatic HIV-positive individuals.

A comparative study of [Leu1]Tuftsin and tuftsin, a natural phagocytosis-stimulating peptide

1. [Leu1]tuftsin was reported to have greater phagocytosis-stimulating activity than tuftsin (Thr-Lys-Pro-Arg). 2. However, a study on inactivation of tuftsin by polymorphonuclear leukocytes (PMNs) demonstrated that leucine aminopeptidase, an ecto-enzyme, located on PMN surface was responsible for this mechanism. 3. Since leucine aminopeptidase is known to cleave Leu more easily than Thr at the N-terminal position of peptides, this suggested to us that [Leu1]tuftsin might then be inactivated by PMNs more easily than tuftsin, and thus this analog might be less active than tuftsin. 4. In addition, many tuftsin preparations used in earlier studies were not fully active, as high-performance liquid chromatography was not available to separate out many contaminating diastereomers. 5. In view of this, we have synthesized and purified [Leu1]tuftsin and compared its phagocytosis-stimulating activity with tuftsin. 6. Our results indicate that [Leu1]tuftsin is not as active as tuftsin in stimulating phagocytosis.

The characteristics of purified HL60 tuftsin receptors

The purification and characteristics of purified HL60 tuftsin receptors are described. Purification was accomplished by affinity chromatography similar to that described earlier, wherein a tuftsin analog Thr-Lys-Pro-Pro-Arg, is covalently linked at the N alpha group to a solid support. The receptor consists presumably of two subunits approximately 66 KDa and 57 KDa. The dissociation constant of the receptor complex is 4.7 X 10(-8) M with 5 X 10(4) receptors per cell. It can form oligomers with an Mr of about 560 KDa suggesting an octomeric structure, assuming the same number of each subunit is associated.

Tuftsin-enhanced thymidine incorporation by murine splenic monocytes

Tuftsin (Thr-Lys-Pro-Arg), a natural immunomodulating peptide originally found to stimulate phagocytosis by polymorphonuclear leukocytes (PMN), is now known to bind to both PMN and monocyte-macrophages, affecting many of their functions. Administration of tuftsin induces leukocytosis in vivo. We have recently observed that while tuftsin remains in the cytoplasm upon binding and internalization in human PMNs, it translocates into the human monocyte nucleus, suggesting that tuftsin may directly affect growth of monocytes. We have therefore examined the effect of tuftsin on [3H]thymidine incorporation in fractions of murine splenocytes to identify a cell population responding to tuftsin. Tuftsin showed the greatest effect in [3H]thymidine incorporation of splenocytes over controls at optimum conditions of 2% fetal bovine serum and 1 microgram/ml of tuftsin. Splenocyte fractionation by Lymphocyte Separation Medium indicated that tuftsin primarily affects the mononuclear cell fraction; further fractionation revealed that tuftsin affects mostly the monocytes that adhered to plastic. We subsequently further purified the splenic monocytes by repeated plastic adhesion and Percoll gradient separation, to show that tuftsin increases [3H]thymidine incorporation of these highly purified monocytes.

Human peritoneal macrophage activity is increased by tuftsin

Peritonitis caused by Candida albicans is a major complication of continuous ambulatory peritoneal dialysis (CAPD). Increasing the activity of the peritoneal macrophages--the predominant cell type found in the peritoneal cavity--may be of great importance in the prevention and therapy of peritonitis. Therefore, the activating effect of tuftsin was studied on human peritoneal macrophages from CAPD patients. Tuftsin induced a biphasic effect on macrophage activity within a range of 2 X 10(-9)-2 X 10(-6) M, with a maximal activity of 2 X 10(-7) M. At this concentration, tuftsin enhanced by twofold cell association with radiolabelled candida (from 2 +/- 0.2 to 4 +/- 0.2 candida per macrophage) and superoxide anion production in response to exposure to candida (from 150 +/- 20 to 300 +/- 20 nmoles/mg). These results suggest the potential use of tuftsin as a therapeutic drug.

Tuftsin: its chemistry, biology, and clinical potential

Tuftsin is a tetrapeptide, Thr-Lys-Pro-Arg, which resides in the Fc-domain of the heavy chain of immunoglobulin G. The peptide originates from a specific fraction of the parent protein through enzymatic processing. Tuftsin possesses a broad spectrum of activities related primarily to the immune system function and exerts on phagocytic cells, notably on macrophages. These include potentiation of various cell functions such as phagocytosis, motility, immunogenic response, and bactericidal and tumoricidal activities. The features of tuftsin, coupled with its low toxicity, make the peptide an attractive candidate for immunotherapy. Tuftsin's capacity to augment cellular activation is mediated by specific receptors that were identified, characterized, and recently isolated from rabbit peritoneal granulocytes. Tuftsin has been chemically synthesized by a variety of techniques, some of which are adequate for large-scale preparations. A multitude of analogs have also been synthesized and extensively studied for structure-function relationships.

The identification of serum tuftsin by reverse-phase high-performance liquid chromatography and mass spectrometry

We describe for the first time a method for unambiguously identifying the phagocytic stimulating tetrapeptide tuftsin from trypsinized human serum. The method consists of separating serum tuftsin by reverse-phase (RP)-HPLC, collecting HPLC fractions corresponding to the synthetic tuftsin retention time, and then subjecting those fractions to mass spectrometry/mass spectrometry (MS/MS) analysis, which provides optimal molecular specificity to the measurement. Although quantification was not the goal, it was estimated that the amount of tuftsin found by RP-HPLC and MS/MS was in the hundreds of nanograms per milliliter.

Protection of mice against Plasmodium berghei infection by a tuftsin derivative

In Plasmodium berghei infections, the mortality rate and parasitaemias were significantly reduced and the mean survival time was considerably enhanced by pretreating the animals with a tuftsin derivative, Thr-Lys-Pro-ARg-NH-(CH2)2-NHCOC15H31. This effect of the modified tuftsin was further increased upon its incorporation in the liposome bilayer. These results indicate that tuftsin and its derivatives may prove useful in enhancing nonspecific host resistance against protozoan infections.

Tuftsin stimulates thyrotropin secretion in rats

Tuftsin acts at the hypothalamus level to stimulate thyrotropin-releasing hormone and thyrotropin secretion in rats.

Enhancement of endogenous xenotropic murine retrovirus expression by tuftsin

The exposure of a high-passage clone of Kirsten sarcoma virus transformed Balb/c (K-Balb) mouse cells to tuftsin (Thr-Lys-Pro-Arg) enhanced the expression of endogenous xenotropic retrovirus. The tetrapeptide increased the expression of virus that was infectious for rat, but not mouse, cells in a concentration-dependent fashion (0.001-1000 micrograms/ml). Increased virus expression could be achieved during short-term incubations (3-4 hr), with maximum enhancement occurring over longer time periods (16-18 hr). The enhancement of virus expression by tuftsin was proportional to the spontaneous release of virus. The infectivity of the enhanced virus was neutralized by goat anti-RLV gp 70 serum. Actinomycin D inhibited the induction of virus, suggesting that enhanced expression required de novo RNA synthesis. Tuftsin stimulated DNA, RNA, and protein synthesis in K-Balb cells during 16-hr incubations. Increased cellular proliferation was also seen at various time periods. The effects observed using K-Balb cells offer an opportunity to study the modulation of gene expression by tuftsin in a fibroblast culture system.

Immunologically active nonapeptide fragment of a proline-rich polypeptide from ovine colostrum: amino acid sequence and immunoregulatory properties

It has been previously found that a proline-rich polypeptide (PRP) isolated from ovine colostrum has a regulatory effect on the immune response. To study the relationship between the structure of PRP and its immunomodulatory properties, the polypeptide was digested by chymotrypsin. Products of the proteolysis were separated by gel filtration and three fractions were obtained: PRP-1, PRP-2 and PRP-3. The activity of the fractions was compared with the activity of the untreated PRP. It was found that PRP-1 was inactive, whereas PRP-2 and PRP-3 showed an activity in the regulation of the immune response assayed by measurement of PFC, and by studying effects on delayed hypersensitivity, formation of autologous rosette-forming cell, and sensitivity of thymocytes to hydrocortisone. The activity of PRP-2 and PRP-3 was comparable to the activity of PRP. The PRP-3 fraction of low mol. wt was further purified and a pure nonapeptide of mol. wt 1000 (PRP-3b) was isolated. The amino acid sequence of PRP-3b was: Val--Glu--Ser--Tyr--Val--Pro--Leu--Phe--Pro. The nonapeptide showed the full spectrum of biological activities of PRP. Comparison of terminal amino acid suggested that PRP-3b was neither the NH2- nor the COOH-terminal fragment of PRP. The amino acid sequence of the nonapeptide indicated that PRP-3b is different from other known immunomodulators.

A biochemical study of the phagocytic activities of tuftsin and its analogues

The effects of tuftsin and one of its analogues (4 [lys]-tuftsin) on phagocytosis of Staphylococcus aureus and Candida albicans were investigated in mice and rabbits. Mice were intravenously or orally administered 1-25 mg of 4 [lys]-tuftsin per kg daily for 4 to 7 days. After the mice were further treated with cyclophosphamide, they were intravenously challenged with C. albicans. More than 50% of the mice infected with C. albicans were killed within 7 days, although only 20% to 40% of those infected with the same microbes after 4 [lys]-tuftsin treatment died. A combination of 4 [lys]-tuftsin and an antibiotic was found to be somewhat more effective than either one alone. Further, pretreatment with 4 [lys]-tuftsin depressed microbial growth in the kidneys of mice bearing S-180 tumors. Rabbit peritoneal macrophages and polymorphonuclear leukocytes were harvested by intraperitoneal injection of liquid paraffin and of glycogen solution, respectively. For in vitro study of phagocytosis, rabbit macrophages of polymorphonuclear leukocytes were incubated in Hanks solution together with S. aureus and the number of survivors was determined. Enhanced engulfing activity of macrophages and increased bactericidal activity of polymorphonuclear leukocytes were shown by the in vitro phagocytosis experiments. It is expected that 4 [lys]-tuftsin will be effective against infectious disease, especially in immunocompromised hosts such as patients with malignant tumors.

Tuftsin analogs for probing its specific receptor site on phagocytic cells

Six new analogs of the phagocytosis-stimulating peptide tuftsin were synthesized with the eventual aim of characterizing and isolating the tuftsin receptor. These analogs can be classified as follows: (a) photoaffinity labelling analogs for the specific covalent attachment to the tuftsin receptor: (b) fluorescent analogs containing either rhodamine or dansyl fluorescent probes for microscopic visualization of the tuftsin receptor; (c) biotin analog for separation and purification of the receptor by affinity methods. In this paper we describe the various synthetic pathways employed to introduce sensitive prosthetic groups into the tuftsin molecule while preserving its biological activity. Activities of the various analogs synthesized as compared to tuftsin in biological and receptor-binding assays are described. All analogs are able to stimulate phagocytosis of the macrophage cell as well as complete specifically for tuftsin binding sites on these cells.

The use of synthetic gamma-chain peptides in the localization of the binding site(s) on human IgG1 for the Fc receptors of homologous monocytes and heterologous mouse macrophages

In an attempt to locate precisely the sites on human IgG responsible for binding to monocytes and macrophages, synthetic peptides representative of sequences of the human gamma-chain have been used as potential inhibitors of human [125I]IgG1 binding to human monocytes and mouse macrophages. One peptide, comprising the sequence of Tyr407--Arg416 of the C gamma 3 domain, showed the same maximum inhibition of [125I]IgG binding to mouse macrophages as unlabelled IgG. Two peptides derived from sequences in the C gamma 2 domain were shown to exhibit limited inhibition of Igg binding to homologous human monocytes.

Does tuftsin alter phagocytosis by human polymorphonuclear neutrophils?

The physiological significance of the putative phagocytosis-promoting peptide, tuftsin, was investigated by measurement of chemiluminescence generated during phagocytosis and by assay of the uptake of radiolabeled bacteria. We found no differences in either assay when we compared serum from splenectomized patients (which purportedly lacks tuftsin) with normal serum. Further, there was no difference when serum from splenectomized patients was employed in the presence of absence of exogenous tuftsin. Similar results were obtained under a variety of conditions, utilizing three different challenge particles with varying particle-cell ratios and serum from 20 different splenectomized patients. These results do not agree with the hypothesis that tuftsin plays a major role in promoting phagocytosis.

Mechanisms controlling differentiation and function of antigen-presenting macrophages

We have briefly reviewed our studies on the mechanisms controlling the differentiation and activation of peritoneal antigen-presenting cells. We demonstrated that the peritoneal population is composed of two main subsets of cells, only one of which participates actively in primary antigen presentation. The latter is missing in athymic mice and seems to differentiate under the influence of the shortlived, cortisone-resistant subpopulation of thymocytes. The maturation of the peritoneal macrophages is subjected also to an additional inducing effect, that of the spleen. Macrophages from splenectomized donors are impaired both with respect to antigen presentation to naive and to primed lymphocytes, and with respect to phagocytosis of "opsonized" bacteria. The mature antigen-presenting cell is subjected to activating signals deriving from the Fc-bound Ig molecule. This is mediated via a tetrapeptide, tuftsin, which is cleaved off the CH2 portion of the Ig and activates the immunogenic effect of the antigen-pulsed macrophage.