Antibodies to Tp67 and Tp44 augment and sustain proliferative responses of activated T cells

We have shown previously that binding of a monoclonal antibody (MAb) to Tp44 molecules increased the proliferation of anti-CD3-activated T cells by causing enhanced IL 2 receptor expression and IL 2 release. We now show that anti-CD5 (Tp67) antibodies have a similar effect under conditions in which monocytes are suboptimally activated or where monocytes are not present. The activity did not depend on antibody isotype or on the precise CD5 epitope recognized. Functional experiments indicated that both IL 2 production and IL 2 receptor expression were enhanced by antibody binding. Anti-Tp67 and anti-Tp44 appear to augment proliferation through distinct mechanisms, because both antibodies together had greater activity than either antibody alone. In neither system is the Fc portion of the antibody required, because F(ab')2 fragments had activity equivalent to that of the intact antibody and were effective at concentrations as low as 10 ng/ml. Fab fragments of anti-Tp67 were active, but Fab fragments of anti-Tp44 had no effect. Anti-Tp67 and anti-Tp44 were able to sustain continuous proliferation of anti-CD3-Sepharose-stimulated T cells for up to 2.5 wk without exogenous IL 2 or feeder cells. These experiments suggest that Tp67 and Tp44 are receptors that play a critical regulatory role in the control of T cell proliferation.

Recombinant interferon-gamma inhibits the B cell proliferative response stimulated by soluble but not by Sepharose-bound anti-immunoglobulin antibody

Recombinant-derived interferon-gamma (reIFN-gamma) was found to inhibit B cell proliferation that was stimulated by soluble goat anti-mouse IgD or goat anti-mouse IgM antibodies, but not that stimulated by Sepharose-bound anti-Ig antibodies. Recombinant IFN-gamma also inhibited the BSF-1-enhanced soluble anti-Ig B cell proliferation but did not block BSF-1 enhancement of Sepharose anti-Ig-stimulated B cell DNA synthesis. Recombinant IFN-gamma concentrations as low as 0.001 U/ml were effective in suppressing the soluble anti-Ig-stimulated B cell proliferative response, and this inhibitory effect could be partially reversed by co-culture with a hybridoma anti-IFN-gamma antibody. Recombinant IFN-gamma appeared not to inhibit action of resting B cells from G0 to early G1, because it did not inhibit the increases in cell size that were stimulated by anti-delta antibody. However, it was effective in partially suppressing the anti-delta-induced increases in expression of B cell surface Ia. For reIFN-gamma to exert its maximum suppressive effect, it had to be added within the first 7 hr after the onset of culture with anti-Ig. Because reIFN-gamma is a lymphokine that can be detected in vivo, we suggest that it may play a key role in influencing physiologic B cell activation that is induced by antigens or immune complex-mediated cross-linking of surface Ig.

Enhanced activation of a T cell line specific for acetylcholine receptor (AChR) by using anti-AChR monoclonal antibodies plus receptors

Immune complex-mediated regulation of the immune response has been studied by using T cell lines and monoclonal antibodies (MAb), both specific for the acetylcholine receptor (AChR). Rat T lymphocytes bearing the W3/25 phenotype and specific for AChR from Torpedo californica have been propagated in vitro for nearly 1 yr. These T cells proliferate in response to optimal concentrations of AChR presented by irradiated syngeneic thymus cells. At suboptimal concentrations of antigen there is little activation of the T cell line. We report here that the addition of small amounts of anti-AChR MAb produces dramatic stimulation of the T cell lines at suboptimal doses of AChR. Enhanced activation depends on the isotype and not the fine specificity of the MAb that are used. The observed phenomenon is antigen specific, and in fact, the immune complexes may actually suppress the proliferative response of irrelevant T cells to some extent. The MAb plus antigen are rapidly bound to the surface of the antigen-presenting cell, which we have shown is the dendritic cell.

Induction of human T lymphocyte motility by interleukin 2

Interleukin 2 (IL 2) is known to have multiple immunoenhancing activities that are related to its ability to promote the proliferation and the expression of effector functions of human T lymphocytes. We investigated the potential of IL 2 to induce human T lymphocyte migration. Unstimulated T cells did not respond to IL 2, but T cells exposed to dextran or phytohemagglutinin did respond to IL 2 concentrations from 0.01 to 10.0 U/ml, with significantly increased migration. This activity could be specifically blocked with anti-Tac antibody. Analysis of T lymphocyte subsets revealed that OKT4+ but not OKT8+ lymphocytes responded to IL 2 in the chemotaxis assay. Checkerboard analysis demonstrated that the IL 2-induced chemoattractant activity was predominantly chemotactic rather than chemokinetic in nature. The activity of IL 2 was compared with that of another chemoattractant lymphokine, lymphocyte chemoattractant factor, which was found to stimulate lymphocyte migration without prior exposure to mitogen, and which was not inhibited by anti-Tac. Our data suggest that the lymphocyte migratory response to IL 2 is under the control of the inducible receptor recognized by anti-Tac in a manner similar to the proliferative response to IL 2, but differs from proliferation in its OKT4+ cell specificity.

Interleukin 2 enhances the natural killer cell activity of acquired immunodeficiency syndrome patients through a gamma-interferon-independent mechanism

Patients with the acquired immunodeficiency syndrome (AIDS) exhibit a variety of disorders of cellular immunity, including a deficient ability to generate cytotoxic T cells and depressed levels of natural killer (NK) cell activity. Interleukin 2 (IL 2) in vitro can markedly augment these depressed immune functions. Because IL 2 can induce the release of interferon-gamma (IFN-gamma) from normal peripheral blood lymphocytes (PBL), and because IFN-gamma may play a role in the regulation of NK cell activity, this study was performed to determine if the IL 2 enhancement of the NK cell activity of patients with AIDS was an IFN-gamma-dependent effect. PBL from eight healthy heterosexual donors and from nine patients with AIDS were studied for their ability to release IFN-gamma in response to IL 2 at a concentration of 100 U/ml. After 60 hr of culture, the PBL of all eight healthy donors produced IFN-gamma with a mean titer of 113 U/ml (range 40 to 320 U/ml). In contrast, the PBL from only two of nine patients with AIDS released measurable amounts of IFN-gamma (40 U/ml each) in response to IL 2 with a mean titer of 13.5 U/ml for all nine. Although the PBL from patients with AIDS were deficient in their capacity to produce IFN-gamma in response to 100 U/ml of IL 2, significant enhancement of NK cell activity could be obtained after only 1 hr of PBL treatment with 10 U/ml of IL 2, with an optimal NK enhancing effect occurring at doses of 50 to 100 U/ml of IL 2. The use of an anti-IFN-gamma monoclonal antibody resulted in complete neutralization of the IFN released from the normal PBL cultured with IL 2, but failed to inhibit the IL 2 enhancement of NK cell activity. Exogenous IFN-gamma exhibited different kinetics of enhancement of NK cell activity when compared to IL 2, requiring substantially more than 1 hr of pretreatment of PBL. These results indicate that the PBL from patients with AIDS usually do not release IFN-gamma when cultured with IL 2, and that IL 2 enhancement of the depressed NK cell activity of these patients may be an IFN-gamma-independent event. These results may have important implications for the therapy of AIDS.

Characterization of the opsonic and monocyte adherence functions of the specific fibronectin fragment that enhances phagocytosis of particulate activators

The functional opsonic and monocyte adherence domains within the 180,000 m.w. opsonic fibronectin fragment (180K-opFnf) that selectively augments human monocyte phagocytosis of particulate activators of the alternative complement pathway were analyzed with Fab fragments of monoclonal anti-fibronectin antibodies BC7, CE9, BD4, AB3, and CPG1, and with fragments of intact human plasma fibronectin derived by cathepsin cleavage and isolated by affinity chromatography. Monoclonals AB3 and CPG1, which recognize epitopes within 40,000 daltons of the carboxy terminus of intact fibronectin, and the cathepsin D-derived, disulfide-linked fragments that contain these epitopes each inhibited the opsonic function of 180K-opFnf. Monoclonals AB3 and CPG1 inhibited monocyte ingestion of rabbit erythrocytes (Er) by 60 and 50%, respectively, when 180K-opFnf was pretreated with 20 micrograms of these monoclonals, but neither monoclonal affected the enhanced monocyte ingestion of Er pretreated with the fibronectin fragment. The pretreatment of Er with 5 micrograms and 40 micrograms of the disulfide-linked, cathepsin D derivatives isolated from high and low affinity heparin fractions, respectively, inhibited the proportion of ingesting monocytes by 60%, but these types of fragments had little effect when concurrently incubated with the opsonic fragment and Er. Monoclonals CE9 and BD4, which recognize epitopes located adjacent to or within the cell-adhesive domain of intact fibronectin, respectively, inhibited the monocyte adherence function of 180K-opFnf, as evidence by their comparable inhibitory effects when present before or after Er were opsonized with 180K-opFnf. When 20 micrograms of monoclonals CE9 and BD4 were each introduced before and after Er were opsonized with 180K-opFnf, monocyte ingestion was inhibited by 60 and 65% and by 51 and 60%, respectively. At 42 micrograms, cathepsin D-derived, non-gelatin-binding, low affinity heparin fragments that contained both BD4 and CE9 determinants or only the BD4 determinant inhibited monocyte ingestion by 53 and 74%, respectively, when concurrently incubated with 180K-opFnf and target Er, but were without effect when used to pretreat Er before the addition of 180K-opFnf. Thus, the inhibitory effects produced by monoclonals AB3 and CPG1 and by cathepsin D-derived, disulfide-linked fragments containing their corresponding epitopes demonstrated that the opsonic domain within 180K-opFnf is immunologically similar to regions within the carboxy terminus of intact plasma fibronectin.(ABSTRACT TRUNCATED AT 400 WORDS)

Enkephalins: immunomodulators

Our original studies of the enkephalins were centered on behavioral stress and brain dopaminergic interactions. More recently we discovered the enkephalins to be immunomodulators as evidenced by their enhancement effects on lymphocyte blastogenesis in mice, increases in the sizes of the thymus or spleen in rodents, and prolongation of survival of BDF1 mice inoculated with attentuated L1210 cells. Finally, in studies of human blood samples from both normal volunteers and cancer patients, the enkephalins were demonstrated to stimulate active T cell rosettes and natural killer cell activities (in vitro). These studies support our hypothesis that, in stress, the enkephalins modulate the effects of steroid hormones on the immune system.

Enhancement of human B cell proliferation by an antibody to the C3d receptor, the gp 140 molecule

The C3d receptor is a specific marker of B lymphocytes. Recently we have shown that C3d receptor activity is carried by a gp 140 membrane antigen. A polyclonal antibody has been prepared by immunizing a rabbit with highly purified gp 140 molecule isolated from membranes of the human B lymphoblastoid cell line Raji and its high specificity was previously demonstrated. We tested the effect of this antibody to the C3d receptor on the B cell proliferative response. Purified B cells from human blood were induced to proliferate by a B cell growth factor (BCGF)-containing partially purified supernatant from activated T cells. The anti-C3d receptor F(ab')2 enhanced the BCGF-dependent B cell proliferation. This effect was dose dependent, was observed in the presence of different concentrations of BCGF and did not correspond to a change in the time course of the response. The anti-C3d receptor F(ab')2 had no mitogenic effect in the absence of T cell supernatant. In contrast the undigested anti-C3d receptor IgG suppressed the BCGF-dependent B cell proliferation. These results emphasize the potentialities of anti-gp 140 F(ab')2 to explore the involvement of the C3d receptor in the regulation of B cell response to T cell products.

Different susceptibilities to cyclosporin A of the mitogenic and potentiating activities of interleukins

A dissociation was observed between the mitogenic and potentiating activities of interleukins (ILs)-1 or -2 in murine thymocyte cultures treated with drugs. The direct mitogenic effects of the ILs were unaffected by cyclosporin A (CsA) at concentrations which abolished the potentiating activities of these mediators, i.e. their synergy with lectins. Conversely to CsA, dexamethasone was more inhibitory to the mitogenic activity of the ILs than to their synergistic reactions with the lectins. The resistance to CsA of the mitogenic activity of IL-1 was unexpected since this response is assumed to be mediated by newly formed IL-2 and CsA inhibits IL-2 production. This resistance was further tested by coculturing thymocytes with the IL-2-dependent CT6 cells; net gains of thymidine uptake by the cocultures were attributed to IL-2 release. Such net gains were observed in cocultures stimulated with IL-1 alone and were relatively resistant to CsA. On the other hand, net gains stimulated by mitogenic lectins, alone or with IL-1, were eliminated by CsA. These results support the notion that IL-1 direct activity on thymocytes is mediated by newly released IL-2 and show that this IL-1 activity is unusual in being resistant to CsA. Low levels of protection against CsA were also observed in cultures potentiated by IL-1: lymphocytes stimulated by lectins or antigens and IL-1 were inhibited by CsA less than lymphocytes stimulated without IL-1. Yet, this partial protection by IL-1 was achieved only at CsA concentrations about 100 fold lower than those resisted by thymocytes directly stimulated by IL-1.

Modulation induction of the T3 antigen by OKT3 antibody is monocyte dependent

We investigated the influence of monocytes on the susceptibility of the T3 antigen on human T cells to modulation induction by OKT3 antibody. In the absence of monocytes, the T3 antigen was only minimally susceptible to modulation. After the addition of 20% monocytes to the culture, however, complete modulation was readily observed. Furthermore, we found that even in the absence of OKT3 antibody, monocytes were able to down-regulate the expression of the T3 antigen, although to a lesser extent. The ability of monocytes to enhance antigenic modulation proved to be a more general phenomenon. Each individual T cell antigen, however, differed in its susceptibility to modulation by antibody, monocytes, or both, thereby establishing its own characteristic pattern. In addition, after complete modulation of the T3 antigen, the addition of monocytes to the culture thereafter had a distinct inhibitory effect on the reexpression of the T3 antigen. Monocyte enhancement of T3 modulation is significantly reduced when using the OKT3 F(ab')2 fragment, as is OKT3 mitogenesis. After pulsing the monocytes with OKT3 antibody before adding them to the culture, T3 modulation became nearly complete even in the absence of added OKT3 antibody. Monocyte-induced modulation proved not to be MHC restricted, thus allowing for comparative analysis of this effect between monocytes and other cell types. A moderate, however, incomplete modulation enhancement was observed with the human monocyte cell line U937 and with Daudi cells. This finding proved to coincide with the distinct ability of these cell lines to bind OKT3 antibody by their Fc receptors, as was the case with monocytes. In contrast, neither Fc receptor binding nor T3 modulation enhancement was observed with the cell lines Cess and G7. In addition, no effective T3 modulation was observed with glutaraldehyde-fixed monocytes. The overall results seem to indicate that effective modulation of the T3 antigen by OKT3 antibody requires the active participation of Fc receptors on monocytes.

A novel anti-Ia monoclonal antibody which specifically enhances the corresponding T cell alloreactivity

An anti-I-Ab monoclonal antibody, designated K14.83-11, was produced in a fusion between SP 2/0 Ag-14 myeloma cells and spleen cells from a B10.D2/n mouse primed in vivo against C57BL/10. Unlike other anti-I-Ab monoclonal antibodies thus far described, K14.83-11 was found to have a combination of features involving specificity, isotype, and function, unique among existing anti-I-A reagents. K14.83-11 exhibited a strong binding to the I-Ab gene product, with only slight cross-reactivity to the I-Ap/q family of allelic products and no reactivity towards I-Ak,d. When analyzed for isotype, K14.83-11 was found to be of a rare IgG3 isotype. With respect to biologic activity, K14.83-11 not only failed to produce the expected inhibition of specific anti-I-Ab T cell reactivity in vitro, but instead produced a striking enhancement of T cell responses against the I-Ab gene product. The possible relationship of IgG isotype and function was suggested when the immunoenhancing effect of K14.83-11 on reactive T lymphocytes was reversed to that of suppression with highly purified F(ab)2 fragments obtained by pepsin digestion.

Response of resting human peripheral blood natural killer cells to interleukin 2

The present study shows that recombinant interleukin 2 (IL-2) purified to homogeneity induces a rapid and potent enhancement of spontaneous cytotoxicity of human peripheral blood lymphocytes. The cells mediating cytotoxicity after 18-h treatment with IL-2 have surface markers of natural killer (NK) cells and are generated from the peripheral blood subset containing spontaneous cytotoxic cells. A parallel production of gamma interferon (IFN-gamma) is induced by recombinant IL-2 (rIL-2), and NK cells appear to be the major producer cells, whereas T cells are unable to produce IFN-gamma under these experimental conditions. However, the kinetics of the enhancement of cytotoxicity are faster than those of IFN-gamma production, and monoclonal anti-IFN-gamma antibodies do not suppress this effect, making it unlikely that the IFN-gamma produced is responsible for the enhancement. The enhancement of NK cell activity induced by rIL-2 precedes any proliferative response of the lymphocytes, which is instead observed in longer-term cultures of both NK and T cells.

Interferon triggers experimental synovitis and may potentiate auto-immune disease in humans

From these data it appears that IFN is capable of stimulating prostaglandin E and hyaluronic acid production by human synovial fibroblasts in vitro and of initiating an inflammatory reaction in animal joints. In chronic arthritis its production may result from persisting viral or other antigenic stimulation. IFN may enhance the immune response and mediate the inflammatory process in the joint. Its role in the pathogenesis of rheumatic and various other autoimmune diseases is undergoing further study.

Muramyl peptides. Variation of somnogenic activity with structure

Sleep-promoting activities of muramyl dipeptide (MDP) (NAc-Mur-L-ala-D-isogln) and the naturally occurring muramyl peptide(s), factor S, have recently been demonstrated. We now have amplified our understanding of structural requirements for somnogenic activity. The effects of several analogs of MDP on rabbit slow-wave sleep are presented and these results are compared to the dose-response relationship for MDP. Some tentative conclusions as to structural requirements for somnogenic activity are presented; most notably, amidation of the free gamma-carboxyl of MDP and several of its analogs resulted in the loss of somnogenic activity. MDP also can induce febrile and immunostimulatory responses. In the present paper, we show that some analogs possess immunostimulatory and pyrogenic activity but not somnogenic activity, thus suggesting that these biological activities of muramyl peptides may, in part, be mediated by separate mechanisms.

Comparative adjuvant activities of Legionella pneumophila and Mycobacterium tuberculosis

Adjuvant activity of heat-killed Legionella pneumophila was demonstrated and compared with that of inactivated Mycobacterium tuberculosis H37Rv. The two species of bacteria were suspended separately in oil and Arlacel A. Bovine serum albumin (BSA) in saline was then emulsified within the respective adjuvants and injected intradermally into guinea pigs. Antibodies to the BSA antigen in the sera of the animals were quantitated with the kinetic-dependent enzyme-linked immunosorbent assay (k-Elisa). Guinea pigs immunized with BSA in adjuvant with killed L. pneumophila produced high titers of anti-BSA antibody, which, on the average, were nearly as high as in those immunized with BSA in complete Freund's adjuvant with M. tuberculosis H37Rv, and which were much greater than in others immunized with incomplete adjuvant, lacking bacteria. Moreover, with a polypeptide hapten, the L. pneumophila evoked as much or more antibody in rabbits as the mycobacterium adjuvant. The effect of the legionella adjuvant upon the cellular immune response was examined using skin tests. For this purpose guinea pigs were immunized with picryl-guinea pig albumin in these adjuvants. 6 weeks later, they were skin-tested with that antigen. They showed reactions which appeared to have immediate as well as delayed components when examined grossly and histologically. Others, immunized with incomplete adjuvant, did not exhibit delayed reactions. Accordingly, heat-killed L. pneumophila acts as a potent adjuvant. Under the circumstances of these experiments, it was as effective as heat-killed M. tuberculosis.

Characterization of hydrogen peroxide-potentiating factor, a lymphokine that increases the capacity of human monocytes and monocyte-like cell lines to produce hydrogen peroxide

A study was made of a lymphokine produced by human T lymphocytes that mediates activation of human monocytes and monocyte-like cell lines, measured by increased production of H2O2. The lymphokine was produced either by stimulation of human nonadherent peripheral blood mononuclear cells with concanavalin A (Con A) or by stimulation of a human T cell line, HSB2, with Con A and phorbol myristic acetate (PMA). When incubated with freshly isolated peripheral blood monocytes for 48 to 72 hr, the H2O2-potentiating factor (HPPF) stimulated increased production of H2O2, measured in a PMA-triggered assay for H2O2 secretion. Because variations occurred in the response of normal blood donors to the HPPF, human monocyte-like cell lines were used as homogeneous and consistently responsive targets for the lymphokine to facilitate biochemical characterization studies of the factor. Two cell lines were studied: HL60, a human promyelocytic cell line, and U937, a human histiocytic cell line. When target cells of either type were incubated in the presence of the HPPF for 48 to 72 hr, they produced increased amounts of H2O2 in a dose-dependent fashion. H2O2 levels were assessed by means of a microassay that measures peroxide-mediated oxidation of phenol red after an oxidative burst triggered with PMA. By using this assay, HPPF was found to have an apparent m.w. of 54,000 and an isoelectric point of 5.5. The bouyant density was determined to be 1.307, indicating that HPPF is a protein. The utilization of cell lines for both the production and assay of HPPF should facilitate the purification of this lymphokine and the subsequent evaluation of its relationship to other lymphokines known to affect macrophage microbicidal and tumoricidal function.

The adjuvanticity of interleukin 1 in vivo

Interleukin 1 (IL 1) was shown to enhance in vivo secondary antibody responses of mice to a protein antigen. The activity was found to be dose and time dependent. The maximum enhancing effect was obtained with 2000 LAF units of IL 1 given 2 hr after the priming dose of antigen. The observation is consistent with the in vitro immunoenhancing activities of IL 1 and strongly suggests that this protein may be an important mediator of host defense responses, as well as of the stimulatory effects of some adjuvants.

Adjuvant effects of a crystalline silica on IgE and IgG1 antibody production in mice and their prevention by the macrophage stabilizer poly-2-vinylpyridine N-oxide

A crystalline silica (standard quartz DQ12 with particle size less than 5 microns) is able to stimulate in Balb/c mice the production of IgE and IgG1 antibody to a single 1-microgram dose of ovalbumin. The adjuvant effects of silica on both IgE and IgG1 antibody production are prevented by pretreatment of animals with poly-2-vinylpyridine N-oxide, a polymer that protects macrophages from the well-documented toxic effects of silica. These results indicate that adjuvanticity of silica is, at least partly, correlated to the damage induced on macrophages.

The primary role of lymphoreticular cells in the mediation of host responses to bacterial endotoxim

Mice that are unresponsive to lipopolysaccharide (LPS) (strain C3H/HeJ) can be rendered LPS-sensitive by the adoptive transfer of bone marrow cells from LPS-sensitive mice (strain C3H/HeN). This model of adoptive transfer was used to evaluate the contribution of lymphoreticular cells to five effects of endotoxin on the host: immunogenicity, adjuvanticity, lethality, induction of interferon, and induction of colony-stimulated factor. C3H//HeJ mice became sensitive to each of these effects after adoptive transfer of bone marrow cells from C3H/HeN mice. The efficacy of transfer was directly proportional to the dose of X-irradiation and inversely proportional to the number of surviving host stem cells. The most effective dose of radiation was 850 rad, and C3H/HeN leads to C3H/HeJx chimeras prepared at this dose were as sensitive to LPS for each parameter tested as were the C3H/HeN donors except for a threefold greater resistance to lethality than LPS-responsive C3H/HeN mice. C3H/HeN mice could also be rendered unresponsive to LPS by the adoptive transfer of C3H/HeJ bone marrow cells. C3H/HeN chimeras were resistant to all of the effects of LPS studied except for the induction of colony-stimulating factor. These results demonstrate that lymphocytes and/or macrophages play a primary role in mediating a number of diverse and seemingly unrelated host responses to endotoxin.

Ribonuclease-sensitive ribosomal vaccine of Pseudomonas aeruginosa

In mice, active protection against Pseudomonas aeruginosa could be induced with two fractions derived from a crude preparation of ribosomes from P. aeruginosa. The two fractions were obtained by gel filtration chromatography of the crude ribosomal preparation on Sepharose CL-2B. In fraction I, less than 1% of the ribonucleic acid (RNA) applied to the column was recovered. Fraction II contained RNA and protein in a ratio of 1.94. The presence of ribosomes in this fraction was confirmed by analysis on a sucrose density gradient. The protection by fraction I was not affected by treatment with ribonuclease; in contrast, incubation of fraction II with ribonuclease completely abolished active protection. Fraction I contained lipopolysaccharide (LPS) as was indicated by the presence of 2-keto-3-deoxyoctonic acid. No LPS was found in fraction II. The adjuvant dimethyl dioctadecyl ammonium bromide enhanced the protection by fraction II; however, immunity by a low dose of fraction I was abolished by dimethyl dioctadecyl ammonium bromide. Protection by fractions I and II appeared to be restricted to the homologous serotype of P. aeruginosa. These results indicate that RNA is required for protection by fraction II. Active protection by fraction I is likely due to LPS.