I. Introduction

Natural control over neoplasia

Studies over the past two decades have revealed the presence of neoplastic cells in cancer patients who were considered cured or who had attained complete remission following successful therapy. Tumor cells detected at a level below the resolution of conventional microscopy have been termed MRD (reviewed in Moss 1999; Faderl et al, 1999, 1999a). When MRD persists asymptomatically for years without any increment in tumor mass, the tumor is thought to be “dormant” (Uhr et al, 1997). Modern sensitive techniques such as flow cytometry, fluorescence in situ hybridization, and polymerase chain reaction have increased the sensitivity of MRD detection. Molecular evidence of residual leukemia cells has been detected in the bone marrow for as long as 9 years following completion of therapy for acute lymphoblastic leukemia (Potter et al, 1993). Low levels of MRD were found in 15 of 17 acute lymphoblastic leukemia patients who remained in complete remission 2-to-35 months after completion of all treatments (Roberts et al, 1997). Long-term persistence of MRD without clinical relapse has been observed in patients who have undergone allogeneic stem cell transplantation for chronic myelogenous leukemia and in patients with acute myeloid leukemia (Chang et al, 1993; Nucifora et al, 1993; Radich et al, 1995; Kenchtli et al, 1998) and childhood leukemia (Vora et al, 1998), suggesting that leukemia cells may survive for more than a decade in a dormant state. “Ultra-late” recurrences of solid tumors have been described over the years (Tsao et al, 1997; Karrison et al, 1999). Recent reports on the detection of MRD in hematological malignancies such as lymphomas as well as in solid tumors (Corradini et al, 1999; Sharp and Chan, 1999; Gath and Brakenhoff 1999; Kvalheim et al, 1999; Maguire et al, 2000) indicate that long-lasting MRD is not disease- or tissue-specific.

The capability to confine tumor cells is not limited to the state of MRD. Asymptomatic occult neoplasms such as prostate cancer have been detected in elderly patients who died of unrelated causes (Gatling 1990), and “disease-specific” fusion gene products including BCR-ABL, BCL2-IgH, MLL-AF4, and the partial tandem duplications of MLL have been detected in healthy individuals who did not develop cancer during the follow-up period (Biernaux et al, 1995; Dolken et al, 1996; Uckun et al, 1998). Thus, neoplastic cells may remain dormant for years (Faderl et al, 1999, 1999a; Estrov and Freedman 1999) while the human organism successfully confines them, keeping them at a “subclinical” level. For this reason, the benefits of therapeutic intervention in patients with MRD remain questionable (Faderl et al, 1999, 1999a, 2000; Estrov and Freedman 1999).

Finally, the spontaneous remission of cancer, though extremely rare (1 in 60,000 - 140,000 cancer cases) (Chang 2000), is a well-established clinical phenomenon that provides additional evidence that the human organism is capable of combating cancer. Spontaneous remission has been reported in leukemia (Bernard and Bessis 1983, Paul 1994; Bhatt et al, 1995; Dinulos et al, 1997; Grundy et al, 2000), malignant melanoma (Barr 1994) and other skin tumors (Barnetson and Halliday 1997), brain tumors (Bowles and Perkins 1999), breast cancer (Jena et al, 2000), lung cancer, and other neoplasms (Kappauf et al, 1997). This phenomenon is not age-restricted or disease- or tissue-specific.

What mechanisms does the human organism recruit to confine neoplasia, and how can they be used clinically? The data on increased tumor frequency in immunocompromised hosts indicate that immune surveillance plays an important role in tumor growth suppression. The sporadic occurrence of both virus-dependent and -independent opportunistic tumors has been reported in immune-deficient patients (Ioachim 1990; Filipovich et al, 1994; Penn 2000) such as those who are immunosuppressed owing to organ or bone marrow transplantation (Penn 1993, Restrepo et al, 1999, Sobecks et al, 1999, Swinnen 2000, Kwok and Hunt 2000, Angel et al, 2000, Rinaldi et al, 2001, Haagsma et al, 2001, Bhatia et al, 2001), severe combined immunodeficiency (McClain 1997; Elenitoba-Johnson and Jaffe 2001), or AIDS (Fiegal 1999; White et al, 2001; Frisch et al, 2001). Perhaps the most compelling data are from patients with AIDS. The incidences of NHL, central nervous system NHL, and Hodgkin’s disease are approximately 100-fold, 3000-fold, and 10-fold higher in AIDS patients than the incidences in the overall population (Straus 2001). Similarly, AIDS patients have an increased risk of developing B-cell and T-cell lymphomas of all types (Biggar et al, 2001). In addition, neoplasms thought not to be immunodeficiency-related or virally induced, such as carcinomas of the rectum, rectosigmoid, trachea, bronchus, lung, skin, connective tissues, brain, and central nervous system have been found in AIDS patients up to 7 times more frequently than in the general population (Gallagher et al, 2000; Phelps et al, 2001; Clarke and Glaser 2001). One may assume that other, slower growing tumors might remain undetected in these patients because of their shortened life span.

Of the two branches of the immune system, cellular and humoral, cellular immunity has been investigated most extensively and utilized clinically (Pardoll 2001). Donor lymphocyte infusion has been utilized to suppress tumor cell re-growth in patients treated with marrow or blood stem cell transplantation (Appelbaum 2001), and ex-vivo-expanded dendritic cells were successfully used in clinical trials in patients with various neoplasms (Baggers et al, 2000). Recent success in using certain mAbs as anticancer agents has re-attracted investigators’ attention to the role of antibodies in antitumor immunity. This article reviews the evidence supporting the hypothesis that the autonomous production of anticancer antibodies is one measure used by the immune system to confine neoplasia and that certain ANAs of different etiologies confer the humoral branch of antitumor immunity.

II. Humoral immunity, ANAs, and cancer

A. Autonomously produced antitumor antibodies

It has been well established that natural antitumor antibodies may be present in healthy individuals (Colnaghi et al,1977, Chow et al, 1981, Colnaghi et al, 1982). The data on the potent in vivo suppression of experimental tumors by natural antibodies (Kerstin et al, 1996) support their possible tumor-preventive role. Normal human serum was shown to contain natural IgM antibodies cytotoxic to human neuroblastoma cells (Ollert et al, 1996; David et al, 1999). Preliminary results of a phase I/II clinical trial showed an effective arrest of neuroblastoma growth in patients who received such antibodies purified from the blood of healthy antibody-positive donors (Schmitt et al, 1999). Nevertheless, influenced by the limited success of tumor-specific antibodies against established tumors in early clinical trials (Jurcic et al, 1996), most investigators remained skeptical about the antineoplastic role of humoral immunity in general. Several factors were blamed for the limited success of mAb therapy: the shedding of the target antigen from the tumor cell surface, the limited capability of the antibodies to penetrate bulky tumors (Jain 1994), the antibodies’ short half-life in the circulation and limited delivery to tumor sites, the inadequate recruitment of host leukocytes bearing constant (Fc) region receptors, the internalization of the target antigens that render the neoplastic cell resistance, and the lack of highly specific tumor antigens (Schnipper and Strom 2001).

Recently, mAbs targeting antigens that are not shed from the surface of tumor cells were shown to have a substantial therapeutic effect: trastuzumab (Herceptin, a mAb against HER-2/neu, a protein overexpressed in breast cancer cells) against solid tumors, and rituximab (Rituxan, a humanized mAb against the B-cell-specific antigen CD20) against hematologic malignancies (Agus et al, 2000; Marshall 2001). The clinical efficacy of these mAbs supports an important anticancer role of humoral immunity. Natural antibodies might be effective as well, particularly in the early stages of tumorigenesis, when increased intratumoral interstitial pressure, which prevents antibody penetration, does not exist.

Though little is known about their targets on tumor cells (Chow et al, 1981; Aoki et al, 1966; Pierotti and Colnaghi 1967; Martin and Martin 1975; Cote et al, 1983), most natural anticancer antibodies are tumor type-specific. Nevertheless, in recent years, we have identified a subset of natural antibodies capable of binding to the surface of a broad spectrum of cancer cells but not normal cells (Iakoubov et al, 1995, 1995a; Iakoubov and Torchilin 1997, 1998). These antibodies are the natural ANAs, which are present in a substantial proportion of healthy rodents and humans, especially, aged (Globerson 1993; Xavier et al, 1995).

B. Possible anticancer activity of age-related and age-unrelated ANAs

Natural autoantibodies are a substantial part of the natural antibody repertoire, which is present throughout the life span of higher mammals (Cote et al, 1983, Daar and Fabre 1981, Guilbert et al, 1982, Dighiero et al, 1983, Iakoubov et al, 1988; review in ref. Avrameas 1991). It was found that the natural autoantibody repertoire of aged mice is drastically different from that of newborns and healthy adults (Sakharova et al, 1986; Iakoubov et al, 1988), with antinuclear specificity being more frequent in the aged (Xavier et al, 1995; Iakoubov et al, 1988). Ten of 11 IgG class monoclonal autoantibodies, derived from the splenocytes of healthy aged non-immunized Balb/c mice, were shown to possess antinuclear activity, whereas no such mAbas could be obtained from newborn or healthy adult non-immunized mice. Autoantibodies, such as anti-thyroglobulin antibodies and ANAs, have repeatedly been found at significantly higher titers in older humans and laboratory animals without overt disease than in younger controls (review in refs. Walford 1974, Globerson 1993). Although, certain natural antibodies have long been suspected to participate in host protection against neoplasia (Aoki et al, 1966; Pierotti Colnaghi 1967; Martin and Martin 1975; Chow et al, 1981; Cote et al, 1983), ANAs of the aged were believed just to reflect some disregulation in the immune system and were not known to have any functional activity (Ben-Yehuda and Weksler 1992). The presence of elevated blood levels of non-pathogenic ANAs is a characteristic feature of the immune system of the aged (Whitaker and Willkens 1966; Cammarata et al, 1967; Siegel et al, 1972; Hallgren et al, 1973; Walford 1974; Wijk 1976; Globerson et al, 1993; Xavier et al, 1995). Based on their binding specificity, it was hypothesized that ANAs of the aged are an important component of the natural autoantibody repertoire and participate in antitumor immunosurveillance (Iakoubov and Torchilin 1997).

The hypothesis was also based on other considerations. Aging is an established risk factor for tumorigenesis. Various immune functions, especially those of T lymphocytes, decline with aging (review in ref. Ben-Yehuda and Weksler 1992). However, implanted tumors grow at a significantly lower rate in aged laboratory animals than in younger ones (Weksler et al, 1990). Although this difference could be attributed to physiological changes caused by aging, such as reduced blood flow and a diminished supply of nutrients, certain immune tumor-suppressor mechanisms upregulated in the aged might compensate for the deterioration of the T-cell immune function. Age-related elevation of ANA in combination with enhanced ADCC mechanisms in the aged (Ziolkowska et al, 1987) might represent such an upregulated immune mechanism.

The hypothesis that certain ANAs of the aged have antitumor activity is strongly supported by the results of direct in vivo experiments, in which mAb 2C5 (monoclonal tumor cell surface-reactive ANA of IgG2a isotype obtained from non-immunized healthy aged Balb/c mouse) effectively suppressed the growth of EL4 T lymphoma in young syngeneic C57BL/6 mice and prolonged survival time in B16 melanoma-bearing mice (Iakoubov et al, 1995, 1995a, Iakoubov and Torchilin 1997). Data on the tumor reactivity and antitumor properties of some ANAs of a different origin (autoimmune ANAs and ANAs induced by immunization) (Walker and Bole 1976; Johnson and Shin 1983; LeFeber et al, 1984; Okudaira et al, 1987; Rekvig et al, 1987; Astaldi Ricotti et al, 1987; Jacob et al, 1989; Prabhakar et al, 1990; Bachman et al, 1990; Sorace and Johnson 1990; Kubota et al, 1990; Bennett et al, 1991; Mecheri et al, 1992; Klinman 1992; Bouanani et al, 1993; Raz et al, 1993; review in refs. Brinkman et al, 1990; Jacob and Viard 1992) seem to favor the hypothesis.

Key properties of ANAs formulated in connection with systemic autoimmune diseases (Monestier and Kotzin 1992; Monestier et al, 1993; Casiano and Tan 1996) may also be relevant to ANAs’ possible involvement in the control of neoplasia. ANAs are directed against certain components of functionally important subcellular particles (Mohan et al, 1993; Monestier 1997), and frequently target autoantigens associated with active cell division and proliferation. Casiano and Tan (1996) further stated that these properties “support the hypothesis that ANAs are driven by subcellular particles such as organelles or macromolecular complexes which might be in an activated or functional state. This hypothesis leads to the central question of how endogenous subcellular particles that are normally sequestered can be released from cells and exposed to the immune system in a manner that renders them capable of driving a sustained ANA response. An emerging view is that apoptosis could be a mechanism by which potentially immunostimulatory self-antigens might be released from cells.” Similar phenomena may take place in the development of a pan-specific anticancer immune response.

Certain additional data discussed below, though not directly related to ANA of the aged support ANAs anticancer properties. Numerous reports provide direct and indirect evidence that ANAs unrelated to age may also possess antitumor activity. First, supportive data were obtained in studies of patients with autoimmune diseases. It was found that the mortality rate from cancer in patients with autoimmune diseases is noticeably less than that in the healthy population (Palo et al, 1977). The proportion of cancer-related deaths in patients with multiple sclerosis is 67% of that observed in the age-matched general population (Sadovnik et al, 1991). Conversely, experimental suppression of autoimmune manifestations in spontaneously autoimmune mice sharply increases the incidence of spontaneous tumors with the most common types being carcinomas, pulmonary adenomas, and lymphomas (Walker and Bole 1976, Russell and Hicks 1968, Walker et al, 1978, Hahn et al, 1975, Morris et al, 1976). These data suggest that certain components of the immune system characteristic of systemic autoimmunity may at the same time have an antitumor function (Walker and Bole 1976). An important feature of systemic autoimmunity is the presence of ANAs (review in ref. von Muhlen and Tan 1995). Although the appearance of tumors in immunosuppressed animals may be connected to the suppression of various immune mechanisms controlling tumors, some autoantibodies from autoimmune mice may have the same specificities as autoantibodies from the aged (Astaldi et al, 1987; Klinman 1992, Bouanani et al, 1993) and may thus be related to tumor control. Data from many investigators indicating the ability of autoimmune ANAs to react with the cell surface (LeFeber et al, 1984; Okudaira et al, 1987; Rekvig et al, 1987; Jacob et al, 1989; Prabhakar et al, 1990; Bachman et al, 1990; Kubota et al, 1990; Bennett et al, 1991; Mecheri et al, 1992; Raz et al, 1993; Rekvig and Hannestad 1997; Koutouzov et al, 1996; review in refs. Brinkman et al, 1990; Jacob and Viard 1992) also support such possibility. It should be emphasized that most of these investigations aimed to study ANAs’ role in autoimmunity and to show that ANAs add to the severity of autoimmune disturbances owing to their ability to react with the cell surface; no special attention was paid to whether there are ANAs that can selectively recognize the surface of tumor cells but not normal cells.

Second, ANAs have been described that appear in response to immunization. In one study, a mAb that was later found to have an antinuclear nature (Sorace and Johnson 1990), was generated by active immunization with leukemia cells. Treatment with this antibody significantly suppressed leukemia cell growth in rats engrafted with 10² - 10³ leukemia cells (Johnson and Shin 1983). The life span of mice bearing Dalton’s lymphoma ascites tumor cells was increased by immunization with conjugates of guanosine-BSA, GMP-BSA, and tRNA-MDSA complex before transplantation of the tumor cells (Kala and Antony 1996). In addition, nucleic acid-reactive antibodies were shown to inhibit the growth of transformed cells in vitro as a result of the higher rate of endocytosis in transformed cells.

Third, a certain positive role of ANAs has been noted in some cancer patients. Multiple reports on circulating ANAs in patients with malignancies have been recently confirmed in patients with lung cancer (Fernandez-Madrid et al, 1999, Blaes et al, 2000) and colorectal carcinoma (Syrigos et al, 2000), and some of these antibodies were associated with a prolonged survival without disease progression. Earlier, antibodies against autologous tumor cell proteins in patients with small-cell lung cancer were shown to be associated with improved survival (Winter et al, 1993).

C. Hypothetical mechanisms of antitumor activity of ANAs

ANAs bind tumor cells, but it is unclear how this binding affects the cells. Unconjugated mAbs may mediate ADCC, induce complement-mediated lysis, or, in some cases, trigger apoptotic cell death. The second and third of these mechanisms probably have no significant role in the antitumor effect of ANAs. Complement-mediated lysis, though crucial in bacterial killing, is not considered a major mechanism in killing eukaryotic cells (Ross 1986). We are also not aware of any ANA able to initiate apoptosis, though ANA binding to surface NSs is known to induce internalization (Koutouzov et al, 1996). The monoclonal ANA 2C5 neither induced complement-mediated cytotoxicity nor affected the proliferation of EL4 and S49 T cell lymphoma cells. However, the monoclonal ANA 2C5 induced significant ADCC in vitro, especially in the presence of exogenous NSs in the culture medium (Iakoubov and Torchilin 1997, 1998). That is why ADCC may underlie the efficacy of ANAs in vivo, along with the ability of ANA-based immune complexes to cause the production of immunostimulatory cytokines (Nakoin and Ralph 1988). The effectiveness of ADCC is related to the isotype of the biologically active Fc part of the immunoglobulin molecule. Differences between various murine IgG isotypes exist; there is no clear pattern, although IgG2a, IgG2b, and IgG3 have been claimed to be the most effective (Herlyn et al, 1985). These isotypes (especially IgG2a) are much less effective in mediating complement-dependent lysis of target cells. Many monoclonal ANAs originating from autoimmune mice, as well as monoclonal ANAs 2C5 and 1G3 originating from healthy aged mice (Iakoubov et al, 1995, Iakoubov and Torchilin 1997), belong to the IgG2a isotype. In addition to ADCC, the monoclonal ANA 2C5 and similar antibodies as well as immune complexes these antibodies can form with free chromatin (NSs) in cancer patients’ circulation might also enhance cancer immunosurveillance by activating some other tumor-specific and non-specific immune mechanisms. Routes for such activation vary from a well-known phenomenon of immune complex-induced release of inflammatory cytokines and proteolytic enzymes to recently described toll receptor-involving events (Leadbetter et al, 2002) and dendritic cells empowerment (Schuurhuis et al, 2002).

III. ANAs, nucleosomes, and cancer

A. Tumor cell surface NSs as ANA targets

Some ANAs with anti-DNA or antihistone specificity recognize the surface of both tumor cells and normal cells (Rekvig and Hannestad 1979, Mecheri et al, 1992, Raz et al, 1993). An ability to recognize tumor cells but not normal cells was found to be characteristic of two monoclonal ANAs of the aged with NS-restricted specificity; their target was surface-bound NSs (well-characterized constituents of nuclear material consisting of DNA and four pairs of histones arranged in a characteristic pattern) (Iakoubov and Torchilin 1997, 1998). One can conclude that NSs are specifically associated with tumor cells and represent a universal molecular target on their surface, whereas free DNA, individual histones, or cross-reactive determinants are associated with the surface of normal cells as well.

Nucleosome binding to the surface of tumor cells might be mediated by a 94 kDa protein on the tumor cell surface membrane, identified as a NS receptor in the human B-lymphoblastoid Raji cell line, monkey CVI cells, and rat pancreas islet tumoral cell line RINm (Jacob et al, 1989). A 50 kDa cell surface NS receptor was also recently claimed to be present on the surface of tumor cells (Koutouzov et al, 1996); this protein was identified as calreticulin by microsequencing (Seddiki et al, 2001). Although no nuclear antigens were found on the surface of freshly isolated normal blood cells (Emlen et al, 1992), the ability of certain normal blood cells to bind NSs in vitro, probably via surface DNA receptors, has been demonstrated (Bell and Morrison 1991; Hefeneider et al, 1992). Therefore, the possibility should be considered that the absence of NSs from the surface of normal cells may be explained by a low concentration of free NSs in the normal circulation rather than by the absence of an appropriate normal cell surface receptor. At the same time, free NSs originating from dead tumor cells are always present in spent media of growing tumor cell lines as well as in cancer patients (Bell and Morrison 1991; Le Lann et al, 1994). They can bind to the surface of DNA- or NS-receptor-bearing living tumor cells, which are the first cells NSs run into after being released from the dead tumor cells in vivo.

In addition to binding to a chromatin-binding receptor on the surface of activated monocytes (Emlen et al, 1992), NSs may bind to the surface of activated T cells via cell surface proteoglycans, such as heparin sulfate, through an electrostatic interaction (usually of low affinity) with basic N-terminal residues of NS-forming histones (Watson et al, 1999). This finding is in agreement with data on the existence of low affinity (Kd 400 nM) receptors (in addition to NS-specific high-affinity receptors [Kd 7 nM]) on the surface of transformed cells (Koutouzov et al, 1996). Recent studies also indicated possible involvement of serum amyloid P component in chromatin binding (Bickerstaff et al, 1999). Thus, the possibility of amyloid P-mediated recognition of NSs by scavenging cells exists.

B. Source of extracellular NSs

Extracellular NSs are known to be present in substantial quantities in tumor cell cultures (Bell and Morrison 1991) as well as in patients with tumors (Le Lann et al, 1994), where they may originate from apoptotic tumor cells, which are present in varying quantities in every developing tumor in vivo (Wyllie 1993). In a dexamethasone-sensitive S49 T cell lymphoma, in which the apoptotic death was initiated among some of the cells, NSs released from apoptotic cells were able to attach to the surface of surviving tumor cells, converting them into better targets for ANAs (monoclonal ANA 2C5 binding was increased 50-fold) (Iakoubov and Torchilin 1998). Nucleosomes appear in apoptotic cells as a result of DNA fragmentation by endonucleases. It is believed that all the degraded intracellular material from an apoptotic cell is endocytosed by living neighboring cells or special phagocytes via special receptors. However, free extracellular nucleochromatin has been observed in vivo under conditions accompanied by massive apoptotic death, such as lupus erythematosis, AIDS, and cancer (Emlen et al, 1994; Licht et al, 2001). Although there are clear evidences that most of circulating NSs in the blood of cancer patients originate from the tumor (Trejo-Becerril et al, 2003), the particular molecular mechanisms of NSs release from the debris of apoptotic cells are not known.

C. Practical value of measuring free blood NSs

Although an elevated level of circulating NSs is not specific for any benign or malignant disorder, some recent data connecting NSs and cancer are of interest. The level of plasma NSs was significantly higher (13- to 18-fold) in patients with primary breast cancer than in individuals without cancer; some other studied cancers also showed much increased level of NSs (up to 25-fold) (Kuroi et al, 2001). The finding that sera of patients with malignant tumors contained considerably higher concentrations of NSs compared with sera of healthy persons (almost 10-fold) and patients with benign diseases was also reported (Holdenrieder et al, 2001). The concentration of NSs in serum was still further increased after chemotherapy or radiotherapy. A subsequent decrease in the level of NSs often correlated with regression of the tumor (Trejo-Becerril et al, 2003; Holdenrieder et al, 2001a).

D. ANAs and tumor-protective role of free NSs.

Reduced NK activity in neoplastic diseases and other disorders characterized by increased (apoptotic?) cell death was reported (Moy et al, 1985; Dunlap et al, 1990). An immunosuppressive effect of apoptotic cells was noticed (Voll et al, 1997) as well as their ability to downregulate the antitumor activity of macrophages (Reiter et al, 1999). The authors of (Reiter et al, 1999) concluded that “given the fact that apoptosis is a consequence of various cancer treatment modalities, this (macrophage impairment) may lead to a suppression of local antitumor reactions and thus actually counteract endogenous immune-mediated tumor defence mechanisms”. Extracellular chromatin fragments inhibited cell killing by NK cells in vitro (Le Lann et al, 1994; Le Lann-Terrisse et al, 1997). It is as if release of NSs into the extracellular space is a tumor self-defense mechanism against NK-mediated lysis. If so, the increased production of NS-specific cytotoxic autoantibodies may be considered an organism's effort to overcome this tumor mechanism. A similar situation has been described in autoimmune diseases (Van Bruggen et al, 1999). Data demonstrating a prolonged time to disease progression and an increased survival rate in cancer patients showing the presence of serum ANAs (Palo et al, 1977; Blaes et al, 2000; Syrigos et al, 2000) seem to support their possible antitumor activity (Figure 1).

However, the real situation may be more complex since a recent study found that both node-negative and node-positive breast cancer patients with high plasma levels of NSs had a significantly better relapse-free survival rate than patients with low levels of NSs (Kuroi et al, 1999). Although plasma concentration of NSs was suggested as a new prognostic factor for breast cancer, the cellular and biological significance of this observation should be further investigated.

IV. ANAs as potential therapeutic agents

Data accumulated over the past decade indicate that the ANAs’ biological role should no longer be considered just a pathogenic moiety in autoimmunity (Ben-Yehuda and Weksler 1992; Isenberg et al,1994). Circulating ANAs are found in about 30% of patients with cancer (Lynn et al, 1976; Idel et al, 1978; Schattner et al, 1983; Silburn et al, 1984; Takimoto et al, 1989). Although some of these patients develop autoimmune syndromes (review in refs. Naschitz et al, 1995; Seda and Alarcon 1995) that complicate anticancer therapy and are clinically troublesome, the presence of ANAs and autoimmune symptoms may be beneficial. For example, patients with chronic myelogenous leukemia who develop autoimmune phenomena as a result of alpha-interferon therapy attain a significantly higher remission rate than those who do not (Sacchi et al, 1995). The antitumor function of ANAs is the first evidence of a beneficial role of ANAs for the host (Iakoubov et al, 1995a; Iakoubov and Torchilin 1997; Torchilin et al, 2001). As antitumor agents, certain ANAs may have a number of advantages compared with conventional antitumor antibodies. First, the ANAs may be effective against a broad spectrum of tumors, since they appeare reactive against the surface of various tumor cells - lymphoid and non-lymphoid, rodent and human. Second, side-effects of ANAs may be minimal, since their natural presence in the blood is not harmful to the host. Third, the underlying antitumor mechanisms may be multimodal and hence more efficient. In other words, if certain ANAs are found to be effective against a broad spectrum of tumors, future optimal treatment regimens may include the monoclonal ANAs in combination with existing tumor type-specific therapies.

Both clinical and laboratory data indicate that the immune system is capable of suppressing neoplastic cell growth and certain autoimmune processes are accompanied by elevated antitumor potential. Should intentional induction of autoimmunity be considered an antineoplastic therapeutic strategy (Pardoll 1999), especially against the background of observations that lupus patients are better protected from cancer (Huges 2001)? The clinical utilization of certain monoclonal ANAs appears to be an attractive option. Since ANAs in aged animals are not associated with any known abnormality, we can also expect that their application as anticancer agents will be not accompanied by adverse reactions. A possibility that tumor immunity can be uncoupled from autoimmune manifestations was demonstrated recently in another experimental system (Weber et al, 1998).

In what setting should ANAs be used? It is possible that the full antitumor potential of this type of antibody can be realized only in the presence of apoptosis-inducing agents that generate the conversion of tumor cell chromatin into NSs and release of these NSs into the interstitium and binding to the surface of surviving tumor cells to make them better targets for the antibodies. As with many other antibodies, the ability of ANAs to eradicate bulky disease may be limited because of tumor penetration problems, their efficacy may lie in fighting small metastases and controlling MRD. Still, clinical trials is the only way to determine if certain ANAs are effective against a broad spectrum of tumors as many experimental data suggest.

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