Tumor-Bound Immunoglobulins:in SituExpressions Of Humoral Immunity. Elsevier; 1977. pp. 95-148. [DOI: 10.1016/s0065-230x(08)60633-8]

Tumor-reactive antibodies evolve from non-binding and autoreactive precursors

The tumor microenvironment hosts antibody-secreting cells (ASCs) associated with a favorable prognosis in several types of cancer. Patient-derived antibodies have diagnostic and therapeutic potential; yet, it remains unclear how antibodies gain autoreactivity and target tumors. Here, we found that somatic hypermutations (SHMs) promote antibody antitumor reactivity against surface autoantigens in high-grade serous ovarian carcinoma (HGSOC). Patient-derived tumor cells were frequently coated with IgGs. Intratumoral ASCs in HGSOC were both mutated and clonally expanded and produced tumor-reactive antibodies that targeted MMP14, which is abundantly expressed on the tumor cell surface. The reversion of monoclonal antibodies to their germline configuration revealed two types of classes: one dependent on SHMs for tumor binding and a second with germline-encoded autoreactivity. Thus, tumor-reactive autoantibodies are either naturally occurring or evolve through an antigen-driven selection process. These findings highlight the origin and potential applicability of autoantibodies directed at surface antigens for tumor targeting in cancer patients.

A history of exploring cancer in context

The concept that progression of cancer is regulated by interactions of cancer cells with their microenvironment was postulated by Stephen Paget over a century ago. Contemporary tumour microenvironment (TME) research focuses on the identification of tumour-interacting microenvironmental constituents, such as resident or infiltrating non-tumour cells, soluble factors and extracellular matrix components, and the large variety of mechanisms by which these constituents regulate and shape the malignant phenotype of tumour cells. In this Timeline article, we review the developmental phases of the TME paradigm since its initial description. While illuminating controversies, we discuss the importance of interactions between various microenvironmental components and tumour cells and provide an overview and assessment of therapeutic opportunities and modalities by which the TME can be targeted.

Non-Specifically Bound Immunoglobulins on Mouse Placental Cells

There seem to be some similarities between escape mechanisms employed by embryos and cancer cells from the maternal or tumor bearer's immune system. In the present study, we investigated the presence of hemagglutinins on placental cells and their relationship to circulating antibodies in mice immunized against sheep erythrocytes before mating. The results obtained demonstrated that placental cells may fix large quantities of maternal immunoglobulins. The hemagglutinins are probably fixed on placental cells by the Fc, since the combining sites of the antibody were free and able to react with the antigen in the « rosette » test. It was also demonstrated that placental tissue may fix up to 1/4 of the maternal circulating antibodies, while only 1/200 of them were transmitted to the fetuses. Some theoretical implications of the presence in the placenta of antibodies not directed against feto-placental antigens are also discussed.

Circulating Immune Complexes in Human Breast Cyst Fluids: Relationship with Intracystic Immunoglobulin and Electrolyte Levels

Circulating immune complexes, the major classes of immunoglobulins and electrolyte concentrations were measured in sixty-two breast cyst fluids aspirated in women affected by gross cystic breast disease. Two main classes of cysts were defined according to the Na/K ratio. Appreciable levels of immunoglobulins were found in almost all samples examined; 66% of breast cyst fluids showed increased levels of immune complexes. A highly significant linear correlation between increased values of immune complexes and immunoglobulin M (p < 0.001) was found in apocrine cysts, characterized by Na/K ratio <3. However, a significant inverse linear correlation was found between positive values of immune complexes and lowered levels of immunoglobulins A (p < 0.001) and G (p < 0.001) in epithelial cysts with Na/K ratio > 3. These data suggest and confirm that the menstrual cycle can also influence or modulate the metabolic activity of human breast cells as a part of the secretory immune system. The relationship between immune complexes and immunoglobulins and electrolyte profiles may provide further knowledge about the immunological features of breast cyst fluid and suggest the possible alteration of immune-response in cystic breast lesions associated with increased cancer risk

The tumor microenvironment: the making of a paradigm

What has been will be again, what has been done will be done again; there is nothing new under the sun

(Ecclesiastes 1:9)

Stephen Paget was the conceptual father of the role played by the Tumor Microenvironment (TME) in tumor progression. The focus of this essay is the developmental phase of the post Paget TME research. Attempts will be made to highlight some of the pioneering work of scientists from the late sixties through the eighties of last century who laid the foundations for the contemporary scientific achievements of TME research but whose ground breaking studies are rarely cited. This review should serve as a small tribute to their great work.

Tumor-microenvironment interactions: dangerous liaisons

The interaction between microenvironmental components and tumor cells is bidirectional. Tumor cells and their products are capable of regulating and altering gene expression in nontumor cells residing in or infiltrating into the microenvironment and exert selective pressures on such cells, thereby shaping their phenotype. Conversely, microenvironmental components regulate gene expression in tumor cells thereby directing the tumor into one or several possible molecular evolution pathways, some of which may lead to metastasis. This review summarizes six instances in which the tumor liaises with different components of its microenvironment. These liaisons result, in most cases, in enhanced tumor progression. In these cases (responses of tumor and nontumor cells to microenvironmental stress, the interaction of the tumor with fibroblasts, endothelial cells and macrophages, the formation of the metastatic niche, and the interaction of the tumor with immunoglobulins) the tumor, directly or indirectly, alters the phenotype of its interaction partners thereby enlisting them to promote its progression. Does the tumor need all these pathways to form metastasis? Is there a hierarchy of interactions with respect to impact on tumor progression? These questions remain open. They may be answered by approaches employed in the analysis of hypercomplex systems.

The tumor microenvironment in the post-PAGET era

The research area of tumor microenvironment is considered, at present, to be an important factor in tumorigenesis and especially in tumor progression. The present mini review is focused on three principles characterizing the nature of the tumor microenvironment. We first discuss the regulatory functions of the tumor microenvironment and the complexity of the combinatorial signaling pathways operating in it. We then address the aspect that the tumor microenvironment incorporates both pro and anti malignancy factors and that a balance between these factors regulates tumor progression. Thirdly we provide evidence that the non-tumor cells in the tumor microenvironment and their products may be different from those of their counterparts residing in non-tumor microenvironments. The conclusion of this mini review is that the tumor microenvironment, by exerting regulatory functions and selective pressures drives cancer cells into one of several molecular evolution pathways thereby determining and shaping their malignancy phenotype.

Receptors involved in microenvironment-driven molecular evolution of cancer cells

Cells, including cancer cells, communicate with their microenvironment via various types of membrane receptors. An important down-stream effect of such interactions is a change in the molecular phenotype of the cells. The microenvironment-driven molecular evolution of cancer cells may induce either growth arrest or death of the cells or alternatively, boost their malignancy phenotype. In this paper we summarize studies from our own laboratory on interactions of cancer cells with microenvironmental ligands via two types of receptors that are not commonly associated with tumour progression i.e. the receptor for the Fc portion of IgG, and Ly-6 proteins of mouse and human origin. We also review information on interactions of tumour-associated chemokines and chemokine receptors with the corresponding microenvironmental factors. We demonstrate how these interactions may drive the molecular evolution of tumour cells and discuss the possible impact of this evolution on tumour progression.

FcR may function as a progression factor of nonlymphoid tumors

Tumor progression is a multistep process involving genetic and epigenetic changes in a transformed clone. Some of these changes may be induced by host factors which may also select for transformed cellular variants with a high ability to survive and propagate. In this article we review studies showing that receptors for the Fc portion of IgG may be expressed on cells from human or animal tumors of nonlymphoid origin. We also review data demonstrating that at least with respect to cells transformed in vitro with Polyoma virus, transformation per se is not sufficient for the induction of Fc receptor expression. We also summarize preliminary data showing that Fc receptor expression is causally involved in conferring a high malignancy phenotype upon transformed cells. Possible mechanisms to explain these observations are discussed.

Does antibody-dependent epitope masking permit progressive tumour growth in the face of cell-mediated cytotoxicity?

It is not clear how immunogenic tumours can grow from small inocula in syngeneic hosts that mount both T-cell and B-cell responses. In this article Lionel Manson argues that antibody coats the tumour cells, protecting them from attack by cytotoxic lymphocytes, and thus permits the tumour to continue to grow and overwhelm the host. These observations may explain the paradox that an immunogenic tumour overwhelms the tumour-bearing host in the face of an ongoing anti-tumour immune response.

Ovarian cancer-associated antibodies recovered from ascites: their use for the isolation of ovarian cancer-associated antigen to produce monoclonal antibodies

Immune complexes (ICs) were recovered from the ascites of a patient with stage IV endometrioid ovarian cancer by sequential precipitation with 33% saturated ammonium sulfate and 2.5% polyethylene glycol 6000 (PEG 6000), followed by affinity chromatography on protein A-Sepharose CL-4B. The IgG-containing ICs were dissociated using 8 M urea, separated by ion-exchange chromatography on Sephadex QAE-50, and subsequently analyzed for purity by immunoelectrophoresis (IEP) and radial immunodiffusion (RID). Recovered antibody was tested for reactivity by immunohistologic techniques against paraffin-embedded tumor tissue and acetone-fixed cell suspensions of epithelial tumors. The antibody which demonstrated ovarian cancer-associated activity was absorbed with antigen extracts of breast, colon, and lung cancers as well as keratin to reduce cross-reactivity. The absorbed endometrioid ovarian cancer-associated antibody (OCAAb) was used to produce an immunoadsorbent column for the recovery of tumor-associated antigens. A mouse monoclonal antibody designated FEN-1 was produced using this antigen-containing fraction, and preliminary screening has demonstrated ovarian tumor-associated reactivity. The use of autologous ICs as reagents for preparing tumor antigen-rich immunogens may provide a valuable tool in the search for tumor-associated antigens.

Increased expression of Fc gamma receptor in cancer patients and tumor bearing mice

In this study we report on some lines of ongoing research performed in our laboratory, in relation to the increased expression of FcR on tumor cells, as well as on cells present in the tumor-bearing host, and its possible role in tumor progression. In a previous study we have shown that a Polyoma virus (PyV)-induced anaplastic carcinoma (SEYF-a tumor) contained an FcR-expressing subpopulation of tumorigenic cells. We tested the effect of in vivo passaging of FcR-expressing and of non-FcR-expressing sub-populations of SEYF-a tumor cells on the expression of FcR, as revealed by the ability of these cells to bind the 2.4G2 monoclonal antibody, which is directed against mouse Fc gamma 2b/gamma 1R. It was found that upon in vivo passaging these two sub-populations became practically identical in their ability to bind anti-Fc gamma R antibody. On the other hand, in vitro passaging of FcR-expressing SEYF-a cells resulted in a gradual decrease in the expression of Fc gamma R. These results, indicating that the expression of Fc gamma R on tumor cells, per se, is dependent on a factor present in the in vivo environment were confirmed using 3T3 cells transformed in vitro by PyV (C) and forming tumors at first injection to mice (CTC). C cultures of various clones did not express Fc gamma R, while CTC cultures (cultures from tumors) became positive. We also detected an increase in the level of a soluble form of Fc gamma 2b/gamma 1R in the circulation of mice bearing PyV induced tumors. This increase paralleled the appearance of palpable tumors. A similar pattern of increase was observed in mice inoculated with the c-H-ras transformed tumorigenic clone 8/F/5, but not in mice inoculated with non-tumorigenic 3T3 cells. Data published by us show that metastatic breast cancer patients had significantly elevated Fc gamma R levels on their peripheral blood mononuclear cells (PBMC). Experiments presented here indicate a direct correlation between increased Fc gamma R levels on PBMC and tumor mass in colon, ovary and lung metastatic carcinoma patients. The possibility that malignantly transformed cells have the potential to cause proliferation of Fc gamma R expressing T cells was tested. It was found that extract derived from r-H-ras transformed 3T3 cells triggers the proliferation of a T cell hybridoma expressing Fc gamma R.

Ovarian tumor antigens: preliminary histological investigation

The immunoglobulin IgG was isolated from the immune complexes obtained from the ascitic fluids of patients with ovarian serous cystadenocarcinoma. The IgG isolates did not react in immunoperoxidase staining of normal ovarian, fallopian tube, or endocervical tissue. In contrast, the IgG isolates did react in immunoperoxidase staining of autologous and allogeneic ovarian cancer tissue. However, the staining was variable in terms of areas of positivity and was completely absent in some serial sections. This variability was also evident in control sections of an ovarian mucinous cystadenocarcinoma that was positive for the carcinoembryonic antigen. In addition, it appears that serous and mucinous ovarian cancers may share common antigenic determinants. These results demonstrate the antigenic heterogenous nature of ovarian cancer.

Inflammatory cells in 256 Walker tumour growing in the stomachs of rats

The present study was undertaken in order to determine the relative number of inflammatory cells in Walker 256 tumour implanted in the gastric mucosa of the rat. Inflammatory cells were found in all tumours, the relative content ranging from 5-21 (mean 12%), which is comparable to findings in human intestinal tumours. Approximately 80% of the inflammatory cells were mononuclear (lymphocytes, plasma cells and macrophages); less than 20% were polymorphonuclear (neutrophils and eosinophils). Both the number of inflammatory cells and the composition of the cell infiltrates changed during the tumour growth. The highest proportion was found on day 6 after implantation. In addition to the inflammatory cells within the tumor parenchyma, a large number of inflammatory cells were found in the tumour stroma. Possible mechanisms for the inflammatory cell reaction within and around the experimental tumours are discussed.

Immunosuppressor control as a modality of cancer treatment: effect of plasma adsorption with Staphylococcus aureus protein A

In tumor-bearing hosts both cellular and humoral tumor-growth-enhancing factors are present. They cause immunosuppression and facilitate the growth of tumors. Very early during tumor growth these factors are either elicited by the tumor cells or induced by the host immunocytes. Among these immunosuppressive agents, circulating immune complexes appear to play a predominant role. They also activate suppressor cell activity. Plasma adsorption of CIC and IgG by protein A of Staphylococcus aureus has been reported to cause tumor regression. Plasma adsorption with protein A-collodion charcoal, protein A-silica, or protein A-Sepharose also induced tumorilytic reactions. Even direct infusion of protein A induced tumor regressions in rat mammary tumors. Recent studies showing tumor regressions following S. aureus Wood 46 plasma adsorption or infusion of normal plasma adsorbed over S. aureus indicate that specific blocking factor removal by plasma adsorption may not be the mechanism for causing tumor destruction. Results indicate that S. aureus plasma adsorption leaches a number of staphylococcal agents. Thus, it appears that staphylococcal agents, protein A, enterotoxin, and other factors are responsible for the induction of reactions leading to tumor destruction. A unified mechanism explaining the results obtained with plasma adsorption using protein A of S. aureus, or S. aureus Wood, or direct protein A infusion, was presented.

Stage-related correlations between immunoglobulins and complement components in preoperative sera from patients with gastric carcinoma

Immunoglobulins A, G and M and complement components C3, C4 and C1-INH were quantitated in sera taken preoperatively from 168 patients with gastric carcinoma. The values were grouped according to stages (pTNM). The concentrations of C4 and C1-INH increased with advancing stage of disease and were above normal mean values in all stages. The concentration of IgG was below the normal mean value among all the patients and with the lowest concentration in stage III. Concentrations of IgA and C3 were above normal means but without significant relation to stages. There was a positive correlation between the concentrations of IgG and C1-INH in sera from patients with stage IV carcinoma, while the same correlation was negative in stages I-III. Although the results varied among the patients within each stage, the profiles of immunoglobulin and complement concentrations are of value in the preoperative staging of the disease.

Immunobiologic staging of patients with carcinoma of the head and neck

For decades immunologic deficiencies have been documented in patients with carcinoma, and many investigators have attempted to utilize this information in determining prognosis more accurately. Determining prognosis more accurately would be most helpful with Stage III patients, since at present there are no useful guidelines as to which Stage III patients would benefit from aggressive surgery and/or radiation therapy and which would not. Assays of cellular immunity--such as skin testing, determining peripheral T-lymphocyte counts, and assessing lymphocyte reactivity--have achieved extremely limited clinical application. These assays are too expensive and/or too difficult to perform in the routine clinical setting. Total lymphocyte counts, however, are readily available in the clinical setting and can be helpful in determining prognosis. Other readily available clinical information, such as age and pretreatment serum immunoglobulin levels, can also be of value in determining prognosis. In this study, logistic regression was utilized to determine the prognostic implications of stage, age, and pretreatment total lymphocyte counts in 132 patients with carcinoma of the head and neck. Although each variable determined prognosis accurately less than 70% of the time, the stage, age, lymphocyte prognostic index (SALPI) accurately predicted outcome at 3 year follow-up 80% of the time (Pr. less than .0001). Discriminant function analysis was utilized to determine the prognostic implications of pretreatment serum immunoglobulins in 243 patients with head and neck carcinoma. A serum immunoglobulin prognostic index (SIPI) was derived which was based on the observations that elevated levels of immunoglobulin A (IgA) indicated a poor prognosis, while elevated levels of immunoglobulin E (IgE) and immunoglobulin D (IgD) indicated a favorable prognosis (Pr. less than .009). The SIPI was not as accurate as the SALPI in determining prognosis; however, when the indices were used simultaneously, concurring indices predicted the outcome of Stage III patients correctly 86% of the time.

Immunoglobulins reactive to carcinoembryonic antigen and their relationship to the antigen in malignant ascitic fluid of ovarian carcinoma

Samples of malignant ascitic fluid from 30 patients with advanced ovarian carcinoma were examined for the presence of IgM antibodies to CEA and PEG-precipitable proteins binding to 125I-CEA. The IgM antibodies to CEA were measured by a solid-phase radioimmunoassay using ovarian CEA. There was no correlation between the level of IgM antibodies to CEA and that of total IgM in the fluid. In 11 of 30 (37%) samples tested, significant amounts of IgM antibodies to CEA were found. The CEA-binding proteins were measured by the ability of ascitic fluid to incorporate 125I-colonic-CEA into PEG-precipitable complexes. In 9 of 39 (30%) samples, the precipitation was significant. There was no association between antibodies to the ABO and Lewis blood group factors and these antibodies to CEA. An inverse relationship was observed between the level of CEA and that of CEA-binding proteins shown by the two assays. When 125I-CEA was incubated with these "positive" samples, a high molecular weight fraction was demonstrated by chromatography. By contrast, in the "negative" samples, there was no incorporation of 125I-CEA. These findings would indicate the presence of CEA-reactive proteins possibly existing as immune-complex-like material in ascitic fluid of some patients.

A modified radio-immune antiglobulin test for the screening of anti-red blood cell antibodies in hybridoma supernatants

A radio-immune antiglobulin test was developed using mouse monoclonal anti-human A red blood cell (RBC) antibodies, and applied to the screening and study of hybridoma supernatants containing anti-human RBC antibodies. The latter antibodies were provided either as known monoclonal antibodies from culture supernatants and ascitic fluids, or as culture supernatants to be screened. The technical feasibility of the test was greatly facilitated by the use of remova-well disposable polystyrene plates, multichannel pipettes, and multiple-plate centrifuge holders. The sensitivity of the test and its relevance for the screening of hybridoma anti-human RBC antibodies is discussed.

Immunoglobulin localization in benign and malignant lesions of the human mammary gland

Using direct imunofluorescence, lesions from 266 human breast specimens were studied for the presence of IgA, IgM, or IgG localization. The lesions included benign elements from 66 subcutaneous mastectomy specimens in which the absence of simultaneous breast malignancy was documented, primary breast carcinomas from 153 mastectomy specimens, and 47 biopsies containing metastatic breast cancer. A statistically significant association of IgA and IgM with benign lesions was contrasted to the association of IgG with malignant lesions. In both primary and metastatic lesions, IgG localization was associated with estrogen-receptor-poor primary cancers as compared with estrogen-receptor-rich primary cancers. Among primary breast cancer patients, IgG localization in the tumor correlated with relative lymphopenia. A shorter disease-free interval was noted in association with IgG localization among the metastatic breast lesions. No statistically significant association between stage of disease and immunoglobulin presence was demonstrable. Moderate-to-severe intraductal epithelial hyperplasias were more often associated with immunoglobulin G localization that were other benign lesions.

Tissue reactivity of IgG eluted from human carcinomas

Mixed haemagglutination with tissue sections was used to study the tissue reactivity of IgG eluted from human carcinomas. IgG eluted from 21 of 29 tumours, bound to the autologous tissue. The binding was mediated through the Fab-portion, and the bound IgG had an intact Fc-portion. Most eluted IgG bound to the autologous tissue, but binding was also seen to other carcinomas of the same type as well as to other types. In addition, the IgG bound to several cancer-free tissues. Accordingly, the eluates of the 21 tumours contained IgG with a broad tissue reactivity. The eluates of a) the remaining 8 tumours, b)normal tissue, and c) liver tissue from patients with non-malignant diseases, contained IgG which did not bind to any tissue. The IgG associated with these tissues was probably non-specifically bound or bound to receptors in vivo.

In situ identification of inflammatory cells in malignant, non-lymphoid human tumours

Inflammatory cells (the term is considered to include lymphocytes, plasma cells, macrophages, mast cells and PMN) were identified and quantitated in sections from human carcinomas. Two types of cellular infiltrates are described. In the stroma surrounding the cancer tissue (peritumoural) and, to a lesser degree, also within the malignant tissue (intratumoural), lymphocytes, plasma cells, macrophages and mast cells dominated, whilst numerous polymorph nucleated cells (PMN) and aggregates of macrophages characterized central tumour necroses. The demonstration of both T-lymphocytes and macrophages in and around the cancer tissue supports the view that a local immune reaction is initiated by the tumour, and the consistent finding of IgG, IgA and IgM plasma cells at the borders of the carcinomas, indicates that plasma cells are also part of an immune response at the tumour site. This view is further supported by the demonstration of an increased proportion of IgG containing plasma cells in the stroma of colon tumours, compared with normal mucosa.

Elution of herpes simplex virus-specific cytotoxic antibodies from squamous cell carcinoma of uterine cervix

There is evidence that carcinoma of the cervix uteri is associated with infection with herpes simplex virus type 2 (HSV-2), and although indirect evidence suggests an aetiological relationship, this remains controversial. One facet of the problem is that complement-dependent cytotoxic antibodies is HSV-2-infected cells have been undetectable or found only in low titre in sera from patients with progressive cervical cancer whereas higher titres have been found where cervical lesions are less advanced. Indeed, cytotoxic antibody titres have been found to rise during the follow-up period after successful treatment. Absorption of these antibodies by the tumour cells has been proposed as a possible reason for low HSV-2 cytolytic reactivity in patients with progressing cervical cancer. This explanation assumes the presence of HSV-2-specific antigen(s) on the tumour cells. We now provide support ffor this assumption with evidence that absorbed HSV-2-specific antibodies can be eluted from cervical cancer tissue.

Fragments and subclasses of IgG in eluates of human malignant tissues

Eluates of 13 malignant tumours were prepared at 56 degrees C using the continuous flow technique. By using immunodiffusion techniques, 50--80 per cent of the IgG detected was found to be of the IgG1 subclass. The ratio of Ig/kappa to Ig/lambda was similar in eluates and in the corresponding extracts, and this ratio was similar to that obtained using pooled human serum. This indicates a normal distribution of IgG subclasses in the eluates. Besides whole IgG, the eluates and corresponding extracts contained fragments of IgG. This was revealed by using sodium dodecyl sulphate electrophoresis (SDS-PAGE). However, since parts of the IgG associated with human malignant tumours can be non-specifically bound, and since fragments of IgG was found in extracts and eluates of normal tissues, although to a lesser degree than in those of malignant tissue, no conclusive evidence was obtained that the malignant tissue could degrade Ig. SDS-PAGE of extracts and eluates of malignant tissues showed 2--3 constant bands not detected in isolated IgG or in extracts and eluates of most of the normal tissues. These bands were not identified.

Antibody response to embryonal carcinoma cells in syngeneic mice

The immunoglobulin (Ig) classes and subclasses of the specific antibodies contained in antisera raised in male 129/Sv mice against cells of a syngeneic clonal line of embryonal carcinoma (F9) have been determined. Cytotoxic activity was found associated almost exclusively with anti-F9 IgM (micro kappa) antibodies. A large part of anti-F9 activity was found associated with IgG1 (gamma 1 kappa and presumably also gamma 1 lambda) antibodies, and was detectable only by direct immunofluorescence. Traces of specific IgG2a and IgG2b antibodies were also found. No IgG3 and IgA antibodies reacting with embryonal carcinoma cells were detectable under these conditions. The serum of F9 tumor-bearing animals had a very similar Ig composition. Furthermore, IgM and IgG1, but not IgG2 antibodies, were detected at the surface of in vivo growing F9 tumor cells.

Placenta-bound immunoglobulins

Low pH eluates were prepared from trophoblasts derived from 8 term human placentas. A qualitative analysis for immunoglobulins revealed the presence of IgG, IgA, and IgM in these eluates. IgC-rich fractions were obtained by DEAE-cellulose chromatography of ammonium sulfate-concentrated eluates. These fractions were able to neutralize, in vitro, the catalytic activity of reverse transcriptases (RT) from several retroviruses. RT from baboon endogenous virus (BEV) seemed to be more susceptible to the neutralizing activity of some eluates. This was in contrast to RT from feline leukemia virus (FeLV) which were neutralized by eluates of leukocytes from chronic myelogenous leukemia. In contrast to previous and present results with purified IgG from leukemic leukocytes, the purified IgG from placenta eluates was incapable of RT neutralization. However, such purified IgG fractions inhibited mixed lymphocyte reactions.